Enzyme- and immunohistochemical methods were used to characterize the leukocyte populations present in the ileal Peyer's patches of sheep foetuses between 68 and 135 d of gestation and particularly in the period around 100 d of gestation, when active lymphopoiesis begins. A wide variety of leukocytes including IgM+, CD5+, CD4+, CD8+ cells, and MgATPase+ dendritic cells were present at an early stage. Groups of IgM+ cells were seen immediately beneath the epithelium as early as 70 d of gestation. Conventional morphometric and computer-assisted morphometric techniques were used to confirm the significant expansion of these cell populations from 90 d of gestation. IgM+ and CD5+ cells were responsible for the vast majority of the increase in cell numbers. It was concluded that a diverse leukocyte population was present at the initiation of active lymphopoiesis in the ileal PP of the sheep foetus and that all members of this population were associated with the emergence of the dome/follicle primordia from which the B-cell follicle develops.