Antibodies to c-fos oncoprotein were produced in rabbits by immunization with synthetic peptides, corresponding to the sequences 6-15 of N-end and 371-380 of C-end of c-fos oncoprotein. C-fos expression was tested with immunoprecipitation and immunoblotting in various transformed cell lines with antibodies to N- and C-decapeptides. It was shown that antibodies to C-terminal decapeptide revealed a c-fos gene product and also some fos-related antigens FRAs 36 kD, 46 kD, 75 kD and 90 kD in rat pheochromocytoma PC-12 cells and mouse carcinoma cell lines MAC-3 and LL. In some cell lines 46 kD FRA was expressed in the absence of p62 c-fos. Besides, different clones of the same cell line cultivated in identical conditions revealed differences in the 46 kD FRA expression. Antibodies to sequence 6-15 of N-end revealed only c-fos products and no FRAs were detected. Therefore FRAs have homology with the c-fos product in the C-terminal region and differ from it in the N-terminal region.