Anti-human thymocyte globulin was raised in four goats and two horses. The in vitro properties were defined, and clinical effects and immunosuppressive efficacy were tested in Macaca monkeys bearing skin xenografts or allografts. All goat and one horse ALG preparation produced significant graft prolongation; one horse ALG showed no in vivo immune suppression. Graft survival correlated well with the rosette-inhibiting activities of the various ALG batches. Goat ALG was well tolerated and produced no local, systemic or anaphylactic reactions; both horse ALG preparations consistently produced local reactions, serum sickness, and, occasionally severe anaphylactic reactions in sensitized and non-sensitized monkeys. Prolonged treatment with goat or horse ALG for up to 10 months caused lymphocyte depletion and plasmacytosis in lymph nodes and spleen but no other apparent pathological changes in a wide variety of tissues studied; infective complications did not occur. Goat IgG was more immunogenic than horse but circulating antibody titres to xenogenic IgG could not be related to clinical responses to ALG, to deposition of xenogenic IgG in kidneys, or to immunosuppressive potency of ALG batches. It was concluded that goat ALG was consistently more potent and less toxic than horse ALG.