The genotoxic effects of benzene (71432) on bone marrow and spleen lymphocytes were investigated in rats using micronucleus and sister chromatid exchange assay systems. Sprague-Dawley-rats were injected intraperitoneally with 220, 440 or 880mg/kg benzene, and killed 24, 48, or 72 hours later. In bone marrow, benzene exposure caused dose related increases in micronucleated polychromatic erythrocytes (MPCEs) at all three times periods. At the 220mg/kg dose level, a higher number of MPCEs was induced at 24 hours and the frequency decreased over time. At 72 hours in the highest dose group, the MPCE frequency was elevated significantly over control levels. In the group treated and followed for 24 hours, the numbers of micronuclei in spleen cells of rats exposed to 220 and 440mg/kg were not significantly different from controls. Benzene caused a significantly higher sister chromatid exchange (SCE) frequency in bone marrow cells compared to controls. The increase in SCE was significant at all dose levels at 24 and 48 hours. By 72 hours, only rats exposed to the highest benzene concentration had significantly increased SCE frequencies. Benzene decreased the polychromatic erythrocyte to normochromatic erythrocyte ratio in rats exposed to 440 or 880mg/kg levels. No effect on the nuclear division index of spleen cells was noted at 24 hours nor was there a significant decrease in the replicative index of bone marrow or spleen cells.