Indirect immunofluorescence (IIP) staining of tissues from human fetuses (ages ranging from 8 to 32 weeks of intrauterine life) with monoclonal antibodies (MoAb) to monomorphic determinants of Ia antigens and HLA-A,B,C antigens has shown that both types of antigens are already detectable in tissues of 8-week-old fetuses. Ia antigens and HLA-A,B,C antigens reach their almost-complete tissue distribution after 32 and 24 weeks of intrauterine life, respectively. The structure of Ia antigens synthesized by fetal thymus cells is similar to that of B-lymphoid cell-derived Ia antigens. Ia antigen-bearing thymic fetal cells can stimulate allogeneic lymphocytes in mixed lymphocyte reactions (MLRs). These reactions are blocked by monoclonal antibodies to monomorphic determinants of human Ia antigens and of HLA-A,B, antigens.