The melanoma determining Tu locus of the teleost Xiphophorus contains an accessory gene, x-erbB*a, which is closely related to the EGF receptor gene family, and is probably oncogenic. x-erbB*a exists in allelic forms that are specific for distinct Tu-loci, and shows high homology to a non-allelic non-oncogenic counterpart x-erbB*i which is transcribed into mRNA of 4.6 kb in non-tumorous and tumorous tissues of fish harboring and lacking Tu. Expression of a 4.0-kb mRNA in tumors (melanoma and fibrosarcoma) of different etiology is strictly correlated with the inheritance of X. maculatus x-erbB*a alleles; transcripts of 8.0 kb were detected in melanoma and carcinoma of fish harboring a certain x-erbB*a of X. variatus. The expression of the putative x-erbB*a transcripts parallels the stage of malignancy of the tumor. The expression of the xiphophorine EGF receptor gene (x-erbB) was detected in almost all tumors, is strongly enhanced in carcinoma, and is positively correlated with the degree of malignancy of melanoma and fibrosarcoma. Some tumors show expression of erbA-related genes. The PDGF receptor mRNA is expressed in all tumors analyzed and shows enhanced expression in malignant tumors of neurogenic, epithelial and mesenchymal origin. Expression of x-pdgf was observed in several cases of melanoma, but more frequently in carcinoma and fibrosarcoma. We conclude that x-erbB*a might be involved in initiation of tumors of different cellular origin and etiology in fish harboring Tu, as well as in the determination of the malignancy of the tumor. Furthermore, we assume that x-erbB*i, x-erbB, x-pdgf and x-pdgf-r play a role in secondary events in tumorigenesis by, e.g., conferring a selective growth advantage to the tumor cells.