Previous reports from our laboratory showed that tetrasialoganglioside GQ1b, when exogenously added, can promote cell proliferation and neurite outgrowth in two human neuroblastoma cell lines, GOTO and NB-1 (Tsuji, S., Arita, M. and Nagai, Y. (1983) J. Biochem. 94, 303-306). To clarify the activity-associated structure of GQ1b, we analyzed the structure-activity relationships using the GQ1b molecule, derivatives of it, and related gangliosides. When the GQ1b molecule was divided into two parts, the ceramide and oligosaccharide moieties, no activity was found with the former, while with the latter the activity could be seen, although the level of activity obtained never exceeded half that of GQ1b itself and an optimal concentration of 100-200 ng/ml of 20-40-times that of native GQ1b (5 ng/ml) was required. Furthermore, the activity of GQ1b was completely abolished by neuraminidase treatment, which converted GQ1b to GM1, so we examined other molecular species of gangliosides having a common gangliotetraose backbone but linked differently with two to four sialic acids (i.e., GD1a, GD1b, GT1a, GT1b, GQ1b and GQ1c). Among them, only GQ1b was found to be active. The results disclosed the interesting fact that deletion of any sialic acid residue from either of the two disialosyl residues of GQ1b results in a complete loss of activity and that the mere existence of the tetrasialosyl structure does not lead to activity; this indicates the absolute necessity of the GQ1b oligosaccharide structure for the expression of activity. For full expression of the activity, both the ceramide and oligosaccharide moieties were necessary. It was also found that the GQ1b oligosaccharide inhibited the activity of GQ1b at a concentration a few times greater than that of GQ1b, suggesting the involvement of a receptor-like mechanism in the action of GQ1b at the cell membrane.