Quasi-elastic laser light scattering is used to examine the cholera toxin mediated agglutination of ganglioside GM1 containing phospholipid vesicles and Gm1-coated polystyrene spheres. We find that the ability of cholera toxin to agglutinate Gm1-containing phospholipid vesicles depends markedly on the lipid composition of the vesicle, with only those composed of short-chain lipids (C14, C16) being appreciably agglutinated. This is interpreted as due to poor mixing of these lipids with the longer chain gangliosides, resulting in lateral separation of the gangliosides in the membrane bilayer. A simple quantitative model, a modification of that developed by von Schulthess et al. [von Schulthess, G. K., Cohen, R. J., Sakato, N., & Benedek, G. B. (1976a) Immunochemistry 13, 955--962], is developed to describe these agglutination processes. Application of this model to the agglutination of Gm1-coated polystyrene spheres by cholera toxin allows an estimate of a minimum value of 4.5 x 10(4) M-1 for the association constant of cholera toxin for its initial Gm1 receptor.