ABSTRACT: Antisera to some simian retroviruses were previously shown to react with human fetal cells. Conversely, it is demonstrated here that an antiserum to syncytialtro-phoblast plasma membrane (TrPM antiserum) possesses complement-dependent cytotoxic activity to five cell lines chronically producing either baboon endogenous virus (BeV) or Mason-Pfizer virus (M-PV), but not to eight virus-free cell lines. However, TrPM antiserum also lysed the A204 and RAJI cells derived from tumors, as well as RSa cells, which are human fibroblasts transformed by SV40 and Rous sarcoma virus. This agrees with the previously described affinity of TrPM antiserum for some cancer tissues. Absorption of TrPM antiserum with A204 cells suppressed reaction with virus-producing cells, indicating a common determinant. Yet, A204 determinants were not shed into culture medium, in contrast to viral antigens. Finally, a proportion of cord blood lymphocyte suspensions and of cultures from embryonic tissues were lysed by TrPM antiserum and the reaction correlated with sensitivity to antiviral sera.