Abstract: Cell surface gangliosides are presumed to play a role in cell growth and differentiation. Using monoclonal antibodies directed against G.sub.D3, a disialoganglioside expressed predominantly by cells of neuroectodermal origin, we have found that G.sub.D3 is expressed by a subpopulation of cells of the immune system including: a) fetal thymocytes in subcortical regions and near vessels, 2) lymph node lymphocytes in interfollicular areas and near vessels, and 3) a small subset of T cells in the peripheral blood. Mouse monoclonal antibodies (two IgGs, one IgM and F(ab').sub.2 fragments) reacting with G.sub.D3 were found to stimulate proliferation of T cells derived from peripheral blood. Proliferation induced by binding to G.sub.D3 could be augmented by exogenous IL-2, PMA, PHA or Protein A.