Human fetal liver was examined during various stages of gestation for the presence of B cells by using immunoglobulin isotype markers and monoclonal B cell antibodies. Frozen sections were studied with the use of single and double staining methods. The B cell monoclonal antibodies used were BA1, which defines both mature and immature B cells; B1, which identifies mature B cells; and B532, which binds to activated mature B cells. The data indicate that both BA1 and mu+ cells are present at 12 wk gestation, and increase in frequency with age. Delta and B1-bearing cells are detected only later in fetal life. Phenotypically identifiable T cells are present at low frequencies in the fetal liver throughout the time period examined (12 to 21 wk). At 12 to 13 wk gestation, the numbers of kappa- and lambda-chain-positive cells are two to three times greater than the number of mu+ cells. Based on morphology and staining with OKM1, these light chain-bearing cells appear to be non-lymphoid, most likely cells of macrophage origin that have phagocytosed maternal IgG. Our results show that the monoclonal antibodies reacting with subsets of B cells in adults can also be used to define distinct subsets of B and pre-B cells in the fetal liver.