Sister chromatid exchanges (SCEs) induced by the antitumor antibiotic mitomycin-C (50077) (MC) were studied in an in-vivo/in- vitro assay system with spleen and bone marrow of male CD1-mice and Chinese-hamsters. Animals were injected intraperitoneally with MC at concentrations of 0.5, 1.0, 2.0, 4.0, and 6.0mg/kg body weight, and bone marrow or spleen cells were assayed for SCEs under in-vivo and in-vivo/in-vitro conditions. In-vivo/in-vitro tests involved culturing cells from animals exposed to the test compound. A dose related response to MC was observed in both bone marrow and spleen cells of mice. An approximately 25 fold increase in SCEs was observed at the highest concentration of MC. MC at the highest concentration caused mitotic suppression in spleen. In mice, under in-vivo/in-vitro conditions, a similar dose response was observed for bone marrow and spleen cells. In hamsters, under in-vivo conditions, MC caused a dose related increase in SCEs in both bone marrow and spleen. An approximately 15 fold increase in SCEs was observed at the highest concentration of MC. Under in-vivo/in-vitro conditions, in hamsters, a dose response in SCEs was observed for both bone marrow and spleen cells. No significant cell cycle delay was observed. The authors conclude that MC induced SCE responses differ among species, tissues, and methods.