The structural proteins of simian sarcoma virus type 1 and its associated virus (SSV) were analysed; in general they were similar to the corresponding components of murine leukemia viruses (MuLV), i. e. glycoproteins of approximately 69.000 and 71.000 Daltons (gp69/71), proteins corresponding to p15(E) and p12(E) which were identified as envelope, and p31, p15, p12 and p10 proteins identified as internal components. As in MuLV, p12 stained reddish with Coomassie blue but a pl5(C) distinct from p15(E) was not clearly identified. Using antisera specific for individual components of MuLV cross-reactions were observed between the proteins pl5(E), pi2(E) and p31 of SSV and MuLV, and to a minor degree also between their respective major glycoproteins. An antiserum reacting strongly, specifically and almost exclusively with the envelope components of SSV gp69/71, p15 (E) and p12 (E) was prepared in a goat by inoculation of the animal’s own cells, previously transformed in vitro with SSV and grown in goat serum. Comparative studies with this antiserum in complement fixation and Ouchterlony tests confirmed the strong antigenic similarities between SSV and gibbon ape lymphoma virus but did not identify any interspecies reactivity.