CI-994 (acetyldinaline) is an investigative oral anticancer drug currently in clinical trials. To characterize the effects of CI-994 on lymphoid tissue, male rats were administered single oral doses at 0 (vehicle control), 10, 23, and 45 mg/kg and killed up to 7 days after dosing for evaluation of white blood cell differentials, bone marrow differentials, lymphoid tissue weights, and selected histopathology of lymphoid tissue. Statistically significant dose-related reductions in blood lymphocytes (CD-3+, CD-4+, CD-8+, CD-20+), monocytes, neutrophils, and bone marrow lymphoid cells were observed in all drug-treated groups on days 1 and/or 3. Statistically significant reductions in bone marrow myeloid cells were also observed on days 1 and 3 at 23 and 45 mg/kg. Complete or partial reversal of most parameters was evident on day 7. Spleen and/or thymus weights were significantly decreased in the groups administered 23 and 45 mg/kg on days 1, 3, and/or 7. Minor reductions in lymphoid organ weights were noted at 10 mg/kg. Minimal to moderate lymphoid depletion of the spleen and thymus was noted on day 3 in animals dosed at 23 mg/kg. In vitro, CI-994 inhibited mitogen-stimulated blood lymphocyte proliferation with a 50% inhibitory concentration (IC50) value of 3 microM. The results demonstrate that CI-994 can effect lymphoid tissue in rats within 1 day of a single oral dose, that effects are generally reversible within 7 days, and that inhibition of lymphocyte proliferation is a sensitive indicator of CI-994 immunotoxicity in vitro.