SUMMARY Rejection of HL-A-identical, sibling donor kidney allografts has been reported in recipients who were negative for cytotoxic antibody before transplantation. This study evaluates the inhibition of macrophage migration in 10 recipients of HL-A-identical, related donor kidney grafts, and correlates the development of the inhibition of macrophage migration with the likelihood of allograft rejection. Two of the 10 recipients had positive macrophage inhibition assays against donor antigen and both subsequently experienced major rejections. In one of these patients, positive changes in cellular immune response preceded clinical evidence of rejection by three days, and in the other patient, cellular immunity was detected before transplantation, suggesting presensitization despite the negative cytotoxic panel. The maerophage inhibition test is a specific, sensitive, and reproducible assay for detection of cellular immunity which is well adapted to serial studies. It provides an accurate means of screening potential donor-recipient pairs for preexisting cellular immune sensitization and offers a method for monitoring rejection.