Gangliosides may regulate the activity of the immune system in vivo, particularly within tissues such as neoplasms or the central nervous system, where they are most abundant. However, the specific mechanisms by which gangliosides modulate immune function remain incompletely understood. We have characterized the effects that brain-derived gangliosides have on specific steps of the T cell activation process in vitro. Gangliosides inhibit T cell proliferation downstream from the early activation events that are bypassed pharmacologically using the combination of a phorbol ester plus a calcium ionophore. These lipids block IL-2 and IFN-gamma gene transcription without inhibiting the production of IL-4 and IL-10 mRNA. This may be accounted for by the ability of gangliosides to prevent the activation of NF-kappaB in mitogen-stimulated T cells. Despite inhibiting IL-2 production, the antiproliferative effects of gangliosides are not reversed by adding supplemental IL-2 to the culture media. This defect persists because gangliosides also block the entry of activated T cells into the cell cycle. In this setting, phosphorylation of the retinoblastoma gene product, a protein whose phosphorylation state is an important regulator of normal cell cycle progression, is prevented. These studies help to define how gangliosides modulate T cell effector function in vitro. They also highlight the fact that certain T cell responses, namely the production of Th2-associated cytokines, are not inhibited by their actions.