O-acetyl GD3 ganglioside is a cell surface molecule of some neural, neural crest and renal cells. Here we show, by using mAbs specific for O-acetyl GD3 (clone 27A) and flow-cytomotric, biochemical or immunological techniques, that it is also expressed at high intensity level on the surface of 49.6% (median) of the CD3+ cells (T lymphocytes), at medium level in 16.2% of the CD16+ (natural killer) cells, at very low level in 51.9% of CD14+ cells (monocytes) and in 6.9% of CD20+ cells (B lymphocytes), but not in other human blood cells. Of the CD4+or CD8* cells, 52.6 or 36.5% respectively were 27A+. Furthermore, 81.6% of the CD45RO+ lymphocytes carried the O-acetyl GD3 ganglloslde. It was not detected in the thymus, although its immediate precursor, the GD3 ganglioside, was present in the meduilary thymocytes, suggesting that O-acetyltransferases are regulated by maturation events taking place in the periphery. The anti-O-acetyl GD3 antibodies induced a strong mitogenic response in cultured peripheral blood mononuclear cells, but not in purified T cells. However, in combination with phorbol myristate acetate the antibodies induced proliferation also in purified T cells, suggesting that protein kinase C priming is needed for this effect. This and the restricted expression of O-acetyl GD3 suggest a functional role for this ganglioside in T cell subpopulations.