Spontaneous blastogenesis of peripheral blood mononuclear cells in recipients of renal allografts was studied by incorporation of 3H-thymidine. Three different levels of reactivity were observed that correlated with the patient's clinical course. Low levels of blast activity (i.e., median 680 dpm/million cells with 95% confidence interval (CI) of 480 to 880 dpm) were observed in pretransplant patients. A moderate elevation (median 3,00 dpm with CI of 2,300 to 5,300 dpm) was noted post-transplantation during quiescent intervals and following the onset of acute reversed rejection. Marked elevations, i.e., values greater than 10,000 dpm frequently were observed within 6 days prior to rejections (median 21,000 with CI of 7,400 to 34,000 dpm) and during sustained rejections (median 14,000 with CI of 8,800 to 19,600 dpm).