Bone marrow (BM) cells from BALB/c mice infected with mixed-sex Schistosoma mansoni cercariae were assessed for their capacity to form in vitro colony forming units of erythroid (CFU-e) and granulocyte/macrophage (CFU-c) cells. At four weeks after infection, substantial BM commitment to granulopoiesis was noted by an increase in CFU-c, with no changes in CFU-e. Elevated marrow granulopoietic activity, although to a lesser extent, was also observed five weeks after either male or female cercarial infection. Preferential differentiation of precursor cells into eosinophilic and/or neutrophilic lineages was observed at weeks four and six post bisexual cercarial infection. After eight weeks of mixed infection, there was a marked suppression of CFU-e formation as compared with uninfected or single sex cercariae-infected mice. These results demonstrate that S. mansoni infection appears to influence either the differentiation capacity or the levels of BM cells at the progenitor cell level, and that maturation of worms and/or oviposition may contribute to changes in the CFU-e and CFU-c cell differentiation.