Cyclins and cyclin-dependent kinases are critically involved in controlling cell cycle progression in virtually all cells. The recent identification of candidate G1 cyclins in mammalian cells has been a major advance in this field, but the exact functions of these cyclins are unknown. The expression of three D-type cyclins (D1, D2, and D3) was investigated in primary human T lymphocytes as these cells were induced to leave G0, traverse G1, and enter S phase by T cell-specific mitogens. G0 phase T cells expressed low levels of cyclin D2, but not cyclin D3. Treatment of these cells with phytohemagglutinin and 12-O-tetradecanoylphorbol-13-acetate in the presence of fetal calf serum resulted in rapid induction of cyclin D2 RNA in early G1 and slower induction of cyclin D3 in late G1. Cyclin D1 was not detected in T cells under any condition tested. Treatment of T cells with hydroxyurea to arrest cells at G1/S did not block induction of either D2 or D3. However, arrest of cells in mid G1 with deferoxamine blocked D3 expression without affecting D2. Cyclosporin A blocked the induction of both cyclin D2 and D3. Polyclonal antisera were prepared in rabbits against both cyclin D2 and cyclin D3 glutathione S-transferase fusion proteins and used to examine cyclin D2 and D3 proteins in [35S]methionine-labeled T cells. Protein levels were found to correlate closely with RNA levels for both cyclins. No detectable histone H1 kinase activity could be precipitated with either cyclin. However, several cellular proteins were observed to coprecipitate with the cyclins, including several proteins that were observed to associate only with D3. These results indicate that striking differences exist in the induction and regulation of two candidate G1 cyclins in human T cells and suggest that these cyclins could participate in multiple cell cycle checkpoints during G0, G1, or S phase.