The phenotypes of lymphocytes infiltrating the epithelium of the jejunal and ileal Peyer's patches in foetal sheep at about 130 days gestation and 2-month-old lambs were examined using indirect immunoperoxidase histochemistry, a panel of monoclonal antibodies and enzyme histochemistry. Computer-assisted morphometric analysis enabled the relative size of reactive areas within epithelia to be estimated. The comparison of the intraepithelial lymphocyte populations associated with structurally developed Peyer's patches of foetal sheep and those of lambs allowed assessment of the impact of extrinsic factors from which the sheep foetus is shielded. The study confirmed the postnatal expansion in the villous intraepithelial lymphocyte population and showed that this expansion involved the CD8 and gamma delta phenotypes. CD4 lymphocytes did not appear in the follicle-associated epithelium until after birth. Unlike the villous epithelium, the follicle-associated epithelium had a high frequency of IgM+ and MHC II+ cells, which was dramatically reduced after birth. This postnatal reduction was particularly prominant in the follicle-associated epithelium of the jejunal Peyer's patch, where the frequency of IgM+ cells fell from 12.4% in foetal sheep to 0.7% in lambs. Double staining for alkaline phosphatase in the jejunal Peyer's patch suggested that clusters of IgM+ cells were associated with M cells.