The development of lymphocytotoxic antibodies was studied in dialysis patients in relation to the number of blood transfusions they had received. About one-half of the patients did not develop cytotoxins in spite of 30 blood transfusions. cytotoxins were often transient, and sometimes even disappeared in spite of repeated transfusions. Anti-HL-A2 was the most commonly found antibody; however, only 34% of HL-A2-negative patients developed cytotoxins when challenged with blood transfusions. The antigens of the second HL-A locus do not elicit a greater cytotoxic antibody response than antigens of the first locus.