Improved success in clinical renal transplantation is dependent upon optimal matching of donor and recipient tissue antigens, and upon more effective and less toxic means of immunosuppression. Variations in the clinical and pathological manifestations of rejection are a result of the individual's response to foreigntissue antigens which are as yet incompletely defined, and perhaps also to individual differences with regard to the effects of immunosuppressive therapy. In addition to leukocyte typing for HL-A antigens, avoidance of presensitization is a crucial problem for a transplant program. Currently employed therapy, including cytotoxic agents and antilymphocyte globulin (ALG), is not always effective and frequently produces serious infectious complications. Improved methods for production and testing of ALG are needed, as well as investigation of other means of tolerance induction.