In vitro cloning assays for hematopoietic myeloid and erythroid precursor cells have been used as screening systems to investigate the hematotoxic potential of environmental chemicals in humans and mice. Granulocyte-monocyte progenitors (CFU-GM) from human umbilical cord blood and from mouse bone marrow (Balb/c and B6C3F1) were cultured in the presence of lead and the benzene metabolite catechol. Erythroid precursors (BFU-E) from human umbilical cord blood were cultured in the presence of lead. The in vitro exposure of the human and murine cells resulted in a dose-dependent depression of the colony numbers. The concentration effect relationship was studied. Results showed that: (1) Based on calculated IC50 values, human progenitors are more sensitive to lead and catechol than are murine progenitors. The dose that caused a 50% decrease in colony formation after catechol exposure was 6 times higher for murine cells (IC50 = 24 micromol/L) than for human cord blood cells (IC50 = 4 micromol/L). Lead was 10-15 times more toxic to human hematopoietic cells (IC50 = 61 micromol/L) than to murine bone marrow cells from both mice strains tested (Balb/c, IC50 = 1060 micromol/L; B6C3F1, IC50 = 536 micromol/L). (2) A lineage specificity was observed after exposure to lead. Human erythroid progenitors (hBFU-E) (IC50 = 3.31 micromol/L) were found to be 20 times more sensitive to the inhibitory effect of lead than were myeloid precursors (hCFU-GM) (IC50 = 63.58 micromol/L). (3) Individual differences in the susceptibility to the harmful effect of lead were seen among cord blood samples. (4) Toxicity of lead to progenitor cells occurred at environmentally relevant concentrations.