In an attempt to asses the nature of rhesus monkey''s social responding to familiar and unfamiliar peers, individuals in various combinations of 4 Ss [subjects] each were rated over a 15-wk. period in the playpen situation on a 24-item inventory of social responding. The sex ratios in the 3 subgroups were not equal. Both the overall analyses which isolated sex differences showed that several categories of behavior occurred with significantly different frequency among familiar as compared to unfamiliar Ss. With minor exceptions, the interactions involving familiar Ss were seen to be more positive than those involving unfamiliar Ss.