SUMMARYThe post-transplant serums of 14 related kidney transplant recipients were studied for the presence of cytotoxin against donor lymphocytes by a cytotoxin tests. Five of the 14 recipients' serums demonstrated cytotoxins; four of these 5 recipients suffered acute homograft rejection. In all 4 individuals the presence of cytotoxin preceded the clinical diagnosis of rejection. Seven of the 14 recipients did not demonstrate serum cytotoxin; only one of these 7 suffered clinical rejection. Three of the 14 recipients had post-transplant serums with suggestive cytotoxin activity. One of these 3 suffered acute rejection several months later which was preceded by serum cytotoxin activity. It is concluded that serum cytotoxin against donor lymphocytes occurs post-transplant in some recipients, and usually precedes acute rejection.