Numerous technologies of powder and dust material processing use such equipment in which material is dumped. One of less known methods of fine material processing is agglomeration, the process carried out mainly in drum granulators. Formation and growth of granules take place in the surface layer of material, so its shape and size determine the process. In horizontal granulators without baffles non-parallel to the axis of the apparatus, the axial motion of particles may be neglected, which enables one to determine behaviour of the bed in the drum on the basis of the material cross-section. In the paper there is presented the attempt to determine the effect of some operating parameters of a drum on the behaviour of granular material placed in it. It is accomplished by defining the values which characterize the shape and location of the bed cross section. Geometry of the model bed cross section was investigated. The bed of granular material was placed in a glass drum of the diameter D-B = 0,24 m and the length L(beta) = 0,30 m at variable operating conditions. The variable parameters were: - rotational speed of the drum n - 10-120 min(-1) (0.116-1.39n(kr)); - filling of the drum with material phi - 10-50%; - state of the inner surface of the drum: - a) smooth glass surface, - b) surface lined with copper wire net O 0.25 mm with mesh 0.6 mm, - c) smooth glass surface with 12 longitudinal baffles 8 mm in diameter. The basic properties of the model material used in the investigation ale given in Table 1. The experiments were carried out on a rig shown schematically in Fig. 3. The bed motion in the drum was recorded by a S-VHS video camera and then analyzed by a computer image analyzer. Examples of the bed images are shown in Fig. 4a and b. On the basis of the recorded images the following was determined: - changes in the contour line of the bed free surface, - changes in the inclination angle of the contour line, - changes in the length of the contour line of the bed free surface. Figures 5a and b show the examples of changes in the contour line of the bed free durface in a smooth drum depending on the rotational speed of the granulator for two selected materials (Pe) and (MKS), and Figs. 6a and b present contours of the free surface for the extreme positions of the bed during swinging on a smooth wall, reported for two rotational speeds of the drum: 46 and 94 min(-1) at drum filling phi less than or equal to 30%. The state of the inner surface of the drum has a significant effect on the bed motion in a rotating drum. Comparison between the contour lines of the bed free surface (Pe) for three states of the inner surface of the drum is shown in Figs. 7a, b and c. The value which characterizes the position of the transverse bed, and indirectly also the circulation of the material layers, is the angle of onclination of the bed free surface determined as in Fig. 8. Comparison of relation beta = f(n) for eight types of material studied at three levels of smooth wall drum filling is shown in Figs. 9a, b and c. Experimental values were approximated by a straight line determined by equation beta = A + Bn. In Table 2 the coefficients A and B are compared with the values of the angle of natural dumping theta, and the angles beta(o) obtained for the maximum deflection of the bed without slipping and rolling of granules on the surface (static friction). Comparison of the inclination angles of the bed free surface for two states of the drum inner surface and two selected materials (Pe and MKS) is shown in Figs. 10a and b. The elevation of the bed determined by the angle beta is defined by friction forces on the bed-drum wall surface border. Distribution of forces acting on the particles on the inner surface of the rotating drum is shown in Fig. 11. Main change in the bed motion is caused by longitudinal baffles installed in the drum. Comparison of the plots of beta = f(n) for three states of the drum surface is shown in Figs. 12a, b and c. The parameter which determines the shape of the bulk material cross section is the length of the contour line of the bed free surface I-s, which illustrates the path of the grain moving on the material free surface. The examples of the relation I-s = f(n) are shown in Figs. 13 and 14. The increase of the length I-s is connected with larger arching of the contour line of the free surface and denotes an increment of the material free surface on which main phenomena determining the formation and growth of agglomerates take place.