The anatomy of haemopoietic cells in human fetal liver was examined using immunohistological techniques on frozen sections of 31 fetuses (10-28 weeks gestational age). The immunohistological findings were consistent with reported cell suspension data. With regard to the location of haemopoietic activity no particular relationship existed between the various haemopoietic cell lineages. A large number of proliferating cells was present; only a few of these were reactive with haemopoietic progenitor cell monoclonal antibodies (MoAb) CD34. A population of haemopoietic cells expressed CD43 antigen (MoAb MT1) alone or together with anti-vimentin MoAb reactivity; this population needs further delineation. Erythropoiesis and myelopoiesis occurred in clusters around sinusoids and portal triad vessels respectively. Lack of MoAb reacting exclusively with early developmental stages of erythropoiesis and myelopoiesis precluded dissection of these lineages. Lymphopoiesis occurred in a loosely scattered pattern without any sign of focal development. Pre-B and B-cell numbers increased with gestational age. Cells expressing markers of more mature B cells (surface IgD, CD35, and CD21) were rare. Also, few cells reacted with mature T-cell markers, but CD7+ cells were obviously present. This expression of CD7 on haemopoietic fetal liver cells suggests that T-cell precursors develop in fetal liver as well as B cells.