We have used antisera against synthetic peptides to identify and characterize a 37,000 dalton v-mos encoded protein (p37mos) in cells transformed by M-MuSV 124. p37mos, a phosphoprotein, comprises only about 0.0005% of total cellular protein in cell lines transformed by M-MuSV 124. NIH 3T3 cells acutely infected with M-MuSV 124, however, contain 30-100-fold more p37mos. These elevated levels of p37mos correlate with striking morphological changes and cell death in the acutely infected cell population. Using the antipeptide antisera, we have extended the analysis of v-mos proteins to include several other MuSV variants that contain a similar v-mos gene to M-MuSV 124. With the exception of P85, the gag-mos fusion protein from ts110 MuSV, the v-mos gene of these variants is expressed as a 35,000-37,000 dalton protein (size depending on the particular virus).