Out of 9 cell lines from methylcholanthrene-induced fibrosarcomas in BALB/c, 4 lines without expression of murine leukemia virus (MuLV) were selected for immunization of three different F1 hybrids with mitomycin C-treated in vivo sarcoma cells. Antibody activity was tested by immune adherence (IA) and protein A (PA) assays after the 8th immunization. Two sarcomas, CMC-11 and CMC-12, produced antibodies detectable by IA in (BALB/c x C3H/He)F1, but no antibodies were detected by PA. (BALB/c x C3H/He)F1 anti-CMC-11 was tested against CMC-11 and three other MuLV- sarcomas as well as against the skin fibroblast (C-11) from a BALB/c mouse, in which the primary CMC-11 sarcoma was induced. CMC-11 line reacted up to 1/80, but other lines were weakly positive or negative. Absorption analysis showed that the antigen defined by anti-CMC-11 was present on CMC-11 as well as on many other sarcoma cell lines, but not on BALB/c thymocytes and spleen cells, nor on leukemia and myeloma cells. Among 5 fibroblast cultures tested, C-14 showed complete absorption. It seems significant that only C-14 produced tumor nodules in BALB/c mice, in contrast to 4 other fibroblast cultures. This suggested that this antibody was directed against antigens present on transformed fibroblasts and sarcoma cells. The specificity of (BALB/c x C3H/He)F1 anti-CMC-12 serum indicated that this antiserum also seemed to detect transformation-related antigens, although it showed a broader specificity with regard to the tissue representation detected.