In 1970, we reported the inhibitory effect of an alcohol extract of calf spleen (S4) on the in vitro proliferation of stimulated lymphocytes (1). The active molecule has been found to be a protein, with a molecular weight of 45,000 (2). Its inhibiting activity in vivo has been shown to be tissue specific and not species specific (1), fulfilling, therefore, the main criterion for a chalone (3, 4). The reduction of the number of hemolytic plaque-forming cells in the spleens of mice immunized with sheep red blood cells and the inhibition of in vitro lymphocyte transformation and proliferation induced by phytohemagglutinin (PHA) or histocompatibility antigens in the mixed lymphocyte culture (5, 6) suggested that this extract could exert an immunosuppressive action. Similar results have been obtained in our laboratory by Kiger in 1971 with a similar thymus extract (7) and later with thymus and spleen extracts by Houck et al. (8).