G(d1a), G(d1b) and G(t1b) gangliosides were dispersed in the following membrane-mimicking systems: (a) homogeneous micelles; (b) mixed micelles with G(m1) ganglioside (which is resistant to the enzyme action), Triton X-100 or bovine serum albumin; (c) small unilamellar vesicles of egg phosphatidylcholine. The effect of dispersion on sialic acid release by Vibrio cholerae sialidase was studied. As reference substrates freely interacting with the enzyme the lipid-free carbohydrates of G(d1a) and 3'-sialosyl-lactose were employed. The apparent V(max.) of the enzyme was, with all the gangliosides, dependent on the type of ganglioside dispersion. It was lowest for homogeneous micelles and mixed micelles with ganglioside G(m1), and increased about 6-fold for ganglioside/bovine serum albumin lipoprotein micelles, 15-fold for mixed-ganglioside/Triton X-100 micelles (optimal molar ratio 1:7.5) and 30-fold for phosphatidylcholine vesicles containing 2.5 mol% ganglioside (this proportion was optimal for enzyme activity on the vesicles). For ganglioside G(d1a), the activity on Triton X-100 mixed micelles and on mixed vesicles was even greater (3- and 6-fold respectively) than that displayed on G(d1a) lipid-free carbohydrate. With each of the used gangliosides the apparent K(m) values were very similar values for homogeneous micelles and vesicular dispersions, but showed marked increases for Triton X-100 mixed micelles, approaching the values exhibited by reference oligosaccharides. Triton X-100 micelles and phosphatidylcholine vesicles did not appreciably alter the kinetics of sialidase action on 3'-sialosyl-lactose and on G(d1a) lipid-free carbohydrate, indicating that the above effects are dependent on the intrinsic characteristics of the membrane-like systems containing gangliosides.