A heterogeneous mixt. of air and aq. carboxymethylcellulose soln. was forced through vertical glass 4 m long tubes (diam. 25.3 and 40.6 mm) to det. the pressure drop and other flow characteristics (size of air bubbles, size of liq. slugs, bubble and liq. slug flow rate). The pressure drop was also calcd. from a math. model based on a set of correlation equations (error 2-25%). The theor. calcd. data agreed well with the exptl. ones.