Xiphophorine fish from wild populations are insusceptible to develop neoplasia. In contrast, certain backcrosses of Xiphophorus maculatus (platyfish) with Xiphophorus helleri (swordtail) as the recurrent parent, produce offspring that develop neoplasia in a Mendelian fashion. We concentrated our research on melanoma. The starting signal for the development of melanoma comes from an accessory v-erbB-related oncogene, x-erbB*, which is highly homologous to the human EGF receptor gene, and is part of a platyfish-specific tumor gene-complex designated as Tu. Normally, the platyfish is protected from its own Tu by Tu-specific regulatory gene systems. The swordtail has neither evolved the Tu-complex nor the regulatory gene systems. Therefore, the backcross procedure dismantles the regulatory gene systems thus permitting Tu-directed melanoma formation. x-erbB* derived from the platyfish, together with the swordtail-derived oncogenes src, sis, pdgf-r, ras, myc, erbA are expressed or overexpressed in the melanoma, and inositol lipid turnover is considerably elevated. x-src and inositol lipid turnover have also been found elevated in the healthy tissues (eg brain) of the tumourous fish. To construct a fish strain which is highly susceptible to mutagenic carcinogens, we introduced a particular regulatory gene, ie an oncosuppressor gene (Bs), into the genome of the animals developing Mendelian inherited melanoma, by introgressive breeding. Bs prevents the strain from germ line-inherited melanoma but, following carcinogen-induced impairment in a somatic cell gives rise to the Tu-directed development of clonal melanoma in a particular fish, x-src activity and inositol lipid turnover are elevated in the tumor but, in contrast to the animals bearing the inherited melanoma, are not elevated in the brain. Promoting carcinogens (tumor promoters) do not induce melanoma in this strain. Similarly, in order to breed a fish strain susceptible to tumor promoters, we introduced a regulatory gene (the oncostatic gene g, golden) coding for a pre-transformational arrest of pigment cell differentiation in the stem cell stage of the fish that develop the Mendelian inherited melanoma. The new strain is incapable of developing melanoma. Its x-src activity and inositol lipid turnover is elevated in the brain indicating that the biochemical processes which were found to be correlated with the hereditary melanoma formation, are running in the new strain without the occurrence of melanoma. Following treatment with tumor promoters that overcome the arrest of pigment cell differentiation, melanoma develops within a very short latent period.