To determine whether c-fos is involved in avian leukosis virus-induced nephroblastoma, 28 tumors from chickens were analyzed for novel fos fragments. DNA from 1 of 16 clonal outgrowths (in chicken 6561) contained novel fos-related EcoRI and KpnI fragments which hybridized to both v-fos and viral probes. Oncogenicity tests using filtered 6561 tumor cell homogenates did not reveal a tumor-inducing transduction of c-fos. We conclude that c-fos is only an occasional target for proviral insertions or new transductions in avian leukosis virus-induced nephroblastoma. The results also identify a polymorphism in c-fos in K28 chickens and demonstrate that unintegrated viral DNA is not a general characteristic of avian leukosis virus-induced nephroblastoma.