Two patients with kidney transplants were prescribed anti-human lymphocyte gamma-globulin (AHLG) as an adjunct immunosuppressive treatment. AHLG was prepared from cultured human lymphocytes an antigen and successive anti-AHLG levels were measured using passive hemagglutination tests during and after the AHLG treatment. Anti-AHLG levels began to increase after 10-14 days of daily AHLG administration. Thereafter, the levels tends to decrease transiently by the further administration of AHLG. The titer rose again after the discontinuation of AHLG administration reaching a plateau which continued for a considerable length of time. Pretreatment levels were reverted after more than three months. The anti-sheep RBC Ab and anti-horse RBC Ab levels followed the same pattern as that seen with anti-AHLG Ab. The anti-AHLG Ab proved to be specific anti-horse gamma-globulin Ab. Alterations in the anti-AHLG levels can thus be used to monitor the optimal dosage and period of administration as well as to predict the anaphylactic reaction due to AHLG treatment. Keeping the anti-AHLG level low is mandatory to maintain good immunosuppressive conditions yet avoid anaphylactic reactions.