The induction of HLA class II antigens on graft tubular cells and IL-2 receptor expression on the infiltrating lymphoid cells was studied in 245 prospective aspiration biopsies taken during the first posttransplant month from 20 human renal allografts with histologically verified acute vascular rejection (AVR). Based on the histological findings, the specimens were categorized into two main groups: biopsies from grafts with features of AVR only, and biopsies with a combination of AVR and acute cellular rejection (ACR). Also in the second group the AVR findings were predominant. Biopsies were further divided into two subgroups, depending on whether the rejection was reversible or irreversible. Evaluation of class II and IL-2R expression was done by indirect immunoperoxidase staining using monoclonal antibodies. The initial posttransplant tubular cell class II expression was low in all 20 grafts, with 5-15% positive tubular cells, and IL-2R expression was negative. All 13 grafts with a combination of AVR and ACR displayed class II induction, closely correlating to the blast response, with 50% positive tubular cells on days 2-7 after the onset of rejection, and declining thereafter back to prerejection level in grafts with reversible rejection. In grafts with irreversible rejection, tubular cell class II expression remained elevated. A similar pattern was observed with regard to IL-2R expression: the IL-2R positive cells disappeared from the grafts with reversible rejection, but they persisted in the irreversible rejections. The same pattern of class II and IL-2R expression was observed in grafts with pure AVR and reversible rejection. Instead, completely different findings were seen in grafts with pure AVR and irreversible rejection: there was neither class II induction on tubular cells nor IL-2R expression on lymphoid cells. The persistant inflammation was dominated by mononuclear phagocytes, and no blast response could be detected during the entire follow-up. These findings demonstrate a close relationship between IL-2R expression and tubular cell class II induction also in AVR, in the majority of cases. On the other hand, the findings in grafts with pure AVR in histology and irreversible rejection suggest that AVR is a heterogenous group of rejections, where different cellular and molecular mechanisms are operating.