Electron microscopy of a filamentous complex of GroEL and GroES has been performed on negatively stained specimens. The filaments have been formed when GroEL and GroES at relatively low molar ratios (e.g., 1:1 to 1:3) are incubated at room temperature in the presence of magnesium and ATP. At higher molar ratio, the symmetrical GroEL-GroES complex predominates. Within the GroEL-GroES filaments, the same structural rearrangements of GroEL were observed as in the ''bulletshaped'' or symmetrical GroEL-GroES complexes. In the absence of GroES, GroEL molecules have no tendency to form linear filaments, The sharing of one GroES by two GroEL molecules might indicate a high plasticity of the GroEL-GroES complex. Preliminary Fourier analysis of filaments of increasing length indicates that they possess an inherent helicity. Using the negative staining-carbon film procedure, two-dimensional (2-D) crystal nucleation induced by ammonium molybdate-polyethylene glycol can create undulatory rows of side-on GroEL molecules, which are able extend as a p2 2-D crystal form, readily distinguishable from the filamentous GroEL-GroES aggregates. Crystallographic image processing indicates that the GroEL 2 x 7-mer itself possesses a dyad screw axis, in accord with existing higher resolution X-ray structure data. (C) 1995 Academic Press, Inc.