Supplementation of rat lymphocyte cultures with plasma from alloxan-diabetic rats produced a dose-dependent suppression of mitogen-induced blastogenic responses. Viability measurements indicated that this inhibition was not due to a direct cytotoxic effect of alloxan-diabetic plasma on rat mononuclear leukocytes in vitro. This inhibition was not explained by hyperglycemia alone and was observed when blastogenesis was induced in vitro by phytohemagglutinin (PHA), concanavalin A (con A), or allogeneic cells. Peripheral blood lymphocytes from alloxan-diabetic rats appeared to be more sensitive than normal cells to the inhibitory effect of diabetic plasma. Heating at 56 degrees for 60 minutes produced only a partial loss of the inhibition by diabetic plasma. Alloxan-diabetic rat plasma promoted an increased accumulation of adenosine 3',5'-monophosphate (cyclic AMP) in mononuclear leukocytes. Insulin (1 and 100 muU./ml.) in vitro enhanced PHA-induced blastogenesis but failed to reverse the inhibition caused by diabetic plasma. Ultrafiltration through a cellulose dialysis membrane with an exclusion size of 12,000 molecular weight did not remove the inhibitory factor(s). These results indicate that a depressed cellular immune response is produced during an insulin-deficient diabetic state. The suppressed in-vitro blastogenic response of lymphocytes from alloxan-diabetic rats appears to involve some circulating inhibitory factor(s) in diabetic plasma. This inhibition may be explained, in part, by the ability of diabetic plasma to elevate cyclic AMP in mononuclear leukocytes.