The effects of IDS and of other agents that raise intracellular cyclic AMP levels on a variety of lymphocyte functions occurring during various phases of the cell cycle were compared. IDS, which inhibits DNA synthesis as a result of increasing cyclic AMP levels, has no effect on cyclic AMP until 12 hr after adding mitogen. Evidence that IDS does not affect unstimulated lymphocytes or lymphocytes exposed to mitogen for less than 12 hr was obtained by showing that elevated cyclic AMP, but not IDS, caused increased early calcium uptake. IDS failed to inhibit RNA synthesis until 8 to 12 hr after addition of mitogen. Although prostaglandin E1 and dibutyryl cyclic AMP inhibited release of lymphotoxin, a lymphokine produced by lymphocytes during the early G1 phase, IDS actually increased lymphotoxin release. IDS was unable to duplicate the effects of other cyclic AMP agents in enhancing mitogen responses of unstimulated spleen and thymus cells via a thymus hormone-like maturational effect. In a comparison of lymphocytes either 3 hr (G0 or early G1) or 20 hr (middle to late G1) after addition of mitogen, only 20-hr lymphocytes were able to absorb IDS. These findings suggest that the cell cycle specificity of IDS may be due to appearance of cell receptors for IDS only during a limited phase of the lymphocyte cell cycle in mature cells, and that this cell cycle specificity may be an important factor regulating IDS activity.