We obtained both graft-infiltrating cells of host origin and proximal tubular epithelial cells (PTEC) of donor origin (using a selective serum-free medium) simultaneously from biopsies of rejecting renal allografts. The identity of PTEC cultures was established with monoclonal antibodies. Major histocompatibility complex class I expression could be upregulated and major histocompatibility complex class II expression induced on PTEC by 24- to 48-hr incubation with 200 U interferon-gamma. Graft-infiltrating cells were shown to lyse PTEC grown from the corresponding biopsy and not PTEC from biopsies from other patients. Therefore the lytic activity appeared to be donor-specific. Preincubation of PTEC with interferon-gamma did not consistently increase PTEC lysis. Lysis by graft-infiltrating cells obtained from four patients could be blocked by target-preincubation with anti-class I monoclonal antibodies, in one case both anti-class I and anti-class II monoclonal antibodies could block PTEC lysis. Blocking could also be obtained with anti-CD3 monoclonal antibody. PTEC lysis occurred only with graft-infiltrating cells cultured from biopsies with cellular interstitial rejection. So, PTEC seem to be a target in renal allograft rejection both in vivo and in vitro. This model system may be useful for further studies of cellular interactions between graft-infiltrating cells and their targets.