Exogenous gangliosides have been shown to exert a regulatory influence on the proliferation and differentiation of several cell lines in tissue culture. The effect of ganglioside GM3 on C-1300 murine neuroblastoma (MNB) cell proliferation and the modulating action of fetal calf serum (FCS) concentration in the culture media have been investigated. MNB cells were cultured in DMEM containing 1, 2.5, 5 or 10% FCS, and incubated with GM3 at concentrations ranging from 1.95 to 500 microM. Cell proliferation was assayed 4 days after the addition of GM3 using the CellTiter 96 Non-Radioactive Cell Proliferation Assay. GM3 inhibited MNB cell proliferation in a dose-dependent manner regardless of the FCS concentration in the culture media. However, the magnitude of this inhibitory effect was inversely proportional to the FCS concentration in the culture media. The addition of albumin to MNB cells cultured in DMEM containing 1% FCS exerted no effect on the antiproliferative action of GM3. FACS cell cycle analyses demonstrated that MNB cultured in DMEM containing 1% FCS had a higher proportion of cells in the G0/G1 compartment when compared to those cultured in 10% FCS. The enhanced response of MNB cells to GM3 observed in 1% FCS, may be due to a preferential action on cells in the G0/G1 stage of the cell cycle. These studies have demonstrated that the ganglioside GM3 inhibited MNB cell proliferation in tissue culture and this effect was modulated by FCS concentration in the culture media. Since protein-binding of GM3 by FCS does not appear to be the primary mechanism by which FCS exerts its antagonistic effects, we hypothesize that this may be due to the opposing action of stimulatory growth factors present in FCS.