The present paper describes a standardized procedure for production and purification of horse antihuman thymocyte IgG (ATG) of high purity (98% IgG) and the in vitro properties of the purified product. Around 50% of the serum IgG was recovered. The ATG preparations were stable, had weak anticomplementary activity, were well tolerated when given i.m. or i.v. to monkeys, and contained no demonstrable Au antigens. The cytotoxicity of anti-Macaca thymocyte IgG on human lymphocytes was 20–25% of the activity against Macaca lymphocytes. Anti-Macaca ATG inhibited phytohemagglutinin (PHA)-induced cytotoxicity of human lymphocytes. Antisera to human thymocytes were raised by two different procedures, one of which employed a priming dose of Macaca thymocytes to conserve human antigen. Prolonged and pulse immunizations with thymocytes were compared in two groups of horses. The opsonizing activity of ATG raised by pulse immunization did not fall below the activity of ATG produced by prolonged immunization. The correlation between the cytotoxic and opsonizing activities of ATG was weak.