Hematopoiesis begins in the liver in the 5th week of gestation, with macrophages being the predominant white blood cell. In contrast, hematopoiesis begins in the bone marrow at the 11th week and neutrophils predominate. It is not clear what factors account for the different pathways the progenitors take in these two organs. We sought to further define these differences using histochemical and molecular techniques. We hypothesized that these organspecific differences were due to the presence or absence of GCSF and MCSF, and their respective receptors. First, we examined long bones and livers from 6 to 23 weeks postconception to verify the presence of neutrophils and monocytes using immunohistochemistry with antibodies directed at myeloperoxidase (MPO) and KP1 (CD68, a monocyte marker). Flow cytometry was used to further characterize monocytes and neutrophils of an 11 wk fetal marrow and liver. To determine the of expression of cytokines, messenger RNAs for GCSF, GCSFR, MCSF, and MCSFR were assayed using RTPCR. We found: 1) MPO positive cells occured in clusters in the fetal marrow. In contrast, MPO positive cells occured singly in the liver parenchyma. MPO positive cells appeared more frequently in the periportal areas as gestation progressed. 2) The majority of MPO positive cells in the liver at 11wks were large, highly granular, and immature appearing. In contrast, the bone marrow contained a continuum from immature granulocytes to band neutrophils. 3) The ratio of MPO to KP1 positive cells in the fetal marrow ranged from 1:5 (11wk) to 1:20(23wk) in the fetal liver. In contrast, the marrow ranged from 0:5 (8wk) to 10:1 (14wk). 4) Flow cytometry confirmed the presence of neutrophils in the 11wk fetal marrow and liver. In contrast to the 11wk fetal marrow, there were no CD14 positive monocytes in the liver. 5) mRNA for GCSFR, MCSF, and MCSFR were present in all livers and bones. mRNA bands for GCSF were present in the bones but not the livers. This study further defines the differences in the production of monocytes and neutrophils in the fetal marrow vs liver. Our RTPCR results suggest that these differences may be related to presence of GCSF in the fetal marrow and absence in the fetal liver.