Leukocyte typing by the methods of agglutination and cytotoxicity was used prospectively to select renal transplant donors in 7 families wherein 1 sibling needed a renal transplant and 3 to 7 healthy siblings volunteered to donate. The selected donor in each case (in addition to meeting medical requirements and the requirement of ABO erythrocyte compatibility) was the one judged most compatible with respect to leukocyte typing. Azathioprine was the only immunosuppressant used in the recipients initially. Steroids were withheld until manifestations of rejection were observed. In 5 cases the donors were judged highly compatible with the recipients with respect to leukocyte typing, and in these 5 cases, manifestations of rejection either were not observed at all or were mild. All 5 grafts are functioning well 7–23 months post-transplant. In 3 of these cases, no episodes of rejection were observed and steroid therapy was never used. In 2 cases the donors were judged only moderately compatible with the recipients, and in these 2 cases, considerably more rejection activity was observed than in the other 5. One patient died 4 months post-transplant with a nonfunctioning graft. The other patient had 1 moderately severe episode of rejection which was reversed successfully with prednisone. This graft is functioning well 2 months post-transplant. From this data and from previous reports it is concluded that leukocyte typing, although still developmental, is useful in the selection of relatively compatible donors, at least for grafts between siblings.