Effects of low level maternal inhalation exposure to benzene (1076433) (Bz) on erythrocytic and granulocytic colony forming cells of fetal, neonatal, and young adult offspring were studied. Swiss- Webster-mice were exposed to 0, 5, 10, and 20 parts per million (ppm) Bz, 6 hours per day from days six through 15 of gestation. Liver, splenic, and marrow cells were taken from 16 day old fetuses and 2 day old neonates. Assays of burst forming unit erythroid and colony forming unit erythroid (CFUE) cells were determined using a modified plasma clot method. Granulocyte macrophage colony forming cells were assayed using lung media. Litter sizes, weights, and number of dead or malformed fetuses were within control limits for all levels of Bz. At 5 and 10ppm, there was a marked increase in number of CFUE, while at 20ppm their number decreased. The majority of litters exposed to 10ppm displayed an increase or a decrease in CFUE numbers. Two day old male progeny exposed in-utero to 20ppm showed increased CFUE numbers. Six weeks after birth, CFUE numbers reached control values, with the exception of males exposed to 10ppm; however, the differences between genders were not statistically significant. Exposure to 20ppm caused increases in the numbers of granulocytic macrophage colony forming cells among 2 day old neonates. Reexposure of 10 week old progeny exposed in-utero to 10ppm Bz resulted in a marked reduction in the number of bone marrow CFUE among male mice. Decreases in splenic granulocytic macrophage counts were observed in air and Bz exposed progeny upon administration of Bz to adults, however, decreases were more severe in Bz exposed progeny. The authors conclude that exposure to Bz in- utero at the current occupational limit induces alterations of the murine hematopoietic system which persists into adulthood.