Some type of immunosuppressive regimen for the host is required for prolonged survival of renal allografts in man. The proportion of transplants functioning may be a rough indicator of the efficacy of the various mutually exclusive regimens. In this study, data on 16 immunosuppressive regimens from 372 kidney transplants performed during the 3-year period prior to 16 March 1965 were analyzed. The proportion of transplants functioning at various post-transplant intervals in each regimen group was estimated using conventional life table methods. A comparison of 8 regimens, each used on 2 groups of recipients, 1 group receiving related and the other unrelated transplants, showed an effect of donor-recipient genetic relationship regardless of the immunosuppressive regimen used. In all cases transplants from unrelated donors tended to have the shorter duration of function. The greatest difference (P < 0.05) was noted with regimen 2 (pre- and post-operative azathioprine plus post-operative steroids). No obvious differences other than genetic relationship were noted between the groups to which this effect could be attributed. The mean ischemic periods in cases with functioning and nonfunctioning transplants from related or unrelated donors were compared within 2 regimen groups. Differences were significant between the related and unrelated functioning groups (P < 0.01) only, which may indicate that ischemic period is not a critical variable in transplant function. Local irradiation tended to show a beneficial effect on transplant function in 5 of 7 regimens paired with respect to its presence or absence. Regimen 8X (post-operative azathioprine, post-operative steroids, Actino-mycin, post-operative local irradiation) seemed to give the highest proportion of functioning transplants with cither related or unrelated donor-recipient combinations. Delayed onset of transplant function was discussed. The highest incidence occurred in unrelated transplants treated with regimens 6X, 7, 7X, and 8X. Duration of transplant function did not appear to be closely related to the period of delay before initial function began. No relationship could be estab.