After injecting mouse lymph node cells added with adjuvant to rabbits by i. v. or i. m. route, 13 groups of anti-lymphocyte sera (ALS) were made. Then there were measured the effect of ALS in vitro on the blastformation rate in one-way mixed lymphocyte cultures (MLC) in addition of PHA and the lymphocytotoxic titer of ALS. In this study, observations were also carried out to determine the blastformation rate of responding lymph node cells obtained from ALS injected mice in MLC with PHA, the PHA-induced cytotoxic effect of lymphnode cells described just above and the effect of administration of ALS on the survival time of skin allografts in mice. The results of the study are briefly summarized as follows. 1. ALS in vitro suppress the blastformation rate in the MLC of mouse lymph node cells in the presence of PHA. 2. ALS in vitro inhibit cytotoxic effect of mouse lymph node cells on target allogeneic cells in the presence of PHA. 3. ALS in vitro suppress the blastformation rate of mouse lymph node cells in the MLC added with PHA, and also ALS that inhibit the PHA-induced cytotoxic effect on the target allogeneic cells, prolong the survival time of mouse skin allograft. 4. The lymphocytotoxic titer of ALS in vitro is not correlated to the effect of ALS administration on the survival time of mouse skin allograft. 5. It has been demonstrated that ALS prepared by i. v. route with adjuvant are generally stronger than ALS prepared by i. m. route in those inhibitory effect on transplantation immunity, on the rate of blastformation of MLC added with PHA and on the PHA-induced cytotoxic effect of lymph node cells. 6. Inhibitory effects of ALS on transplantation immunity have better correlation with the decreasing levels in blastformation rate of MLC following addition of these ALS and the decreasing degrees of the PHA-induced cytotoxic effect by addition of these ALS than with these lymphocytotoxic titers.