Quantitative data obtained from the study of a complex heteroantisera in an immunogenetically controlled renal allotransplant system has been presented. Antithymus serum is a potent immunosuppressive agent, potentially nephritogenic by mechanisms of “planted antigen” and antigen-antibody complex deposit and is of many specificities. Animals treated with the heterosera show immune elimination rates when given trace radio-labeled amounts of the foreign protein antigen. Organ localization data obtained by isotopic and fluorescent tracers are presented and it has been demonstrated that a site of action and immunosuppression is at the target organ, probably due to graft binding of the heterologous antibody. Thus sensitization pathways or the efferent limb of allograft destruction may be altered. Until more information is available from the study of this type of material, it would seem that clinical trial be approached with much caution.