New chemical synthetic methods have permitted the synthesis of a spectrum of glycosphingolipid molecular species, some of which are not naturally occurring. Here we have studied a number of chemically synthesized gangliosides for immunosuppressive activity, using a human in vitro specific antigen (tetanus toxoid)-induced assay of the cellular immune response. Chemically synthesized GM3 and GM4 had the same high degree of immunosuppressive activity as did natural GM3 and GM4 gangliosides, verifying that inhibition is intrinsic to the ganglioside molecules and not caused by other molecules sometimes found in natural preparations (e.g., proteins). Studies of modified molecular species of GM3 and GM4, also prepared by chemical synthesis, have shown the influence of certain structural details upon the immunosuppressive activity of gangliosides: (i) the inverse relationship between fatty acyl chain length and immunosuppressive activity is extended to even shorter chain lengths, with the synthetic gangliosides d18:1-C2:0-GM3 and d18:1-C14:0-GM3 being more immunosuppressive than d18:1-C18:0-GM3 and d18:1-C24:0-GM3; (ii) hydroxylation of the fatty acyl group decreases immunosuppressive activity; (iii) substitution of an S-glycosidic bond for an O-glycosidic bond in the sialic acid ketosidic linkage in GM4 does not alter its activity; and (iv) modifications of the sialic acid group variably influence immunosuppressive activity, since KDN-GM3 and -GM4 ganglioside analogues, which contain a 3-deoxy-D-glycero-D-galacto-2-nonulopyranosonic acid in place of N-acetylneuraminic acid, retain activity, while other modifications such as 8-epi-GM3, and to a lesser extent 9-deoxy-GM3, reduce immunosuppressive activity.(ABSTRACT TRUNCATED AT 250 WORDS)