Using a panel of monoclonal antibodies, immunohistological analysis was performed on frozen sections taken from 14 peritransplant renal biopsies and 42 biopsies taken 6±2 days posttransplantation. The following parameters were examined: tubular expression of HLA-DR, DP, and DQ and infiltration with T lymphocytes and macrophages. Of the 42 posttransplant biopsies, 26 were diagnosed as rejecting and 16 as nonrejecting according to clinical and histopathological criteria. HLA-DR antigens were strongly expressed on 8 of 14 peritransplant biopsies, 23 of 26 rejecting biopsies and 13 of 16 nonrejecting biopsies. Tubular expression of HLA-DP and DQ was weak or absent.In the rejecting biopsies there was a significantly increased infiltrate of T lymphocytes of all phenotypes and of macrophages when compared with the nonrejecting biopsies. Graft outcome was invariably favorable in the nonrejecting group, with no graft losses in the first posttransplant year. There were 4 graft losses in the rejecting group, all due to rejection, and further analysis revealed that all 4 had macrophage-dominated infiltrates in their early allograft biopsies. We conclude that immunohistological analysis of early allograft biopsies provides an accurate prognosis of subsequent graft acceptance or rejection and that early macrophage infiltration is a poor prognostic sign.