The effect of time of agent administration, via intraperitoneal injection, on the yield of SCEs in bone marrow cells of male B6C3F1 mice was determined for cyclophosphamide (CP), 7,12-dimethylbenz[a]anthracene (DMBA) and mitomycin C (MMC). Animals were treated with several doses of each carcinogen/mutagen at 3 different treatment times: -1, +1 and +8 h in relation to the onset of BrdUrd administration. The results of these studies indicate that the optimal treatment time for inducing a maximal SCE response is agent-specific. For CP, the slope of the SCE response was greatest at the +8 h treatment time while the maximal response for DMBA occurred at the -1 h treatment time. For MMC, the slope of the SCE response was independent of treatment time and of the method of bromodeoxyuridine administration (intravenous infusion vs. tablet implantation) but dependent on the laboratory conducting the study (Brookhaven National Laboratory vs. Oak Ridge Associated Universities). Based on the results of these studies, the +1 h acute treatment time is considered optimal for the in vivo cytogenetic evaluation of suspect chemicals for genotoxic activity when bone marrow is used as the target cell population.