The presence and distribution of B cells and other early leucocyte populations are described in the spleen of fetal lambs from 40 to 134 days of gestation (length of gestation 150 days). Computer-assisted morphometric analysis and flow cytometry were used to quantify the early predominance of B cells in mid-gestation. B cells appeared at about 48 days and increased in number to occupy over 20% of the spleen area at 77 days. All spleens were collected on their respective livers and at no stage did the livers contain more than a few IgM-positive (+) cells, which were usually close to blood vessels. Two-colour flow cytometry demonstrated that only 1-2% of IgM+ cells expressed CD5 at 81 days. Beyond 77 days, with the expanding presence of T cells, the percentage of area occupied by IgM+ cells declined to stabilize at about 7% during late gestation. The conventional organization of the splenic white pulp was observed from 90 days along with 5' nucleotidase-positive primary follicles. Double staining technique using immunohistochemical methods demonstrated that IgM+ cells were proliferating in the spleen from as early as 51 days and that clusters of proliferating IgM+ cells were prominent between 60 and 77 days. The results of the present study suggest that during the ontogeny of fetal lambs the spleen is a site of B-cell development or expansion before colonization of the ileal Peyer's patch and the subsequent generation of the preimmune antibody repertoire.