Mouse and Chinese hamster models have been extensively used for assessing the cytogenetic effects of environmental carcinogens and mutagens. However, there is little information on comparative analysis of chromosomal damage in these species under in vivo and in vivo/in vitro (culturing of cells from animals exposed to the test compound) systems. To obtain such information, mice and Chinese hamsters were injected with varying concentrations (0.5-6.0 mg/kg) of mitomycin C, an antineoplastic drug. The bone marrow and spleen cells were analyzed for the number of sister chromatid exchanges (SCEs) under in vivo and in vivo/in vitro conditions. The results indicated a dose-related SCE response that varied with species, tissues, and assay conditions. The mouse cells appeared more sensitive to the effects of mitomycin C than did Chinese hamster cells. In general, the SCE frequencies were relatively higher under in vivo conditions than under in vivo/in vitro conditions in both species. The spleen cells had higher SCE values than bone marrow cells under in vivo/in vitro conditions in both species. These differences may be related to the pharmacokinetic properties of the drug in different species and tissues, to treatment conditions, or to the repair capabilities of the cells. This study also indicates the usefulness of recently established bone marrow and spleen in vivo/in vitro cell cultures for comparative cytogenetic analysis.