Monoclonal antibodies were used to analyse adrenal, pancreas, thyroid, and gut samples from human fetuses (14-19 weeks estimated gestational age; EGA) for the presence of class II major histocompatibility complex (MHC) antigens (HLA-DR, DQ, DP) by immunohistochemistry. In the adrenal definitive and fetal cortex, HLA-DR+, DP-, DQ- cells were clearly demonstrated. These DR+ cells were identified, phentotypically, as predominantly tissue macrophages and a small population of CD45R+, IgM+ lymphoid cells. Within the pancreas, numerous cells throughout the tissue were strongly DR+ but DQ-; DP+ cells were not observed until 17 weeks EGA. Using a double-labelling procedure, minor proportions of these DR+ cells were identified as macrophages or as (CD19+) B cells, while endocrine and endothelial cells were negative. Throughout the thyroid, small numbers of DR+ macrophages and small lymphoid cells were detected, although the thyroid epithelial cells were DR-, DP-, and DQ-. Large numbers of DR+, DP+, DQ- cells were observed in the stomach wall and mucosa. In the intestine, DR+, DP+, DQ+ cells positive for all MHC class II loci products were abundant throughout the lamina propria and lymphoid aggregates. The class II antigens appeared in the proportion DR greater than DP greater than DQ and expression was most prevalent in the mid-gut region. A small proportion of epithelial cells of the villi along the gut were weakly DR+ but DQ-, DP-. These results show that DR+ cells in fetal endocrine tissue are mainly 'passenger leucocytes' and that, in contrast to recent reports concerning normal adult tissue, the adrenal and pancreatic endothelial cells are DR-.