We have found that 1 of 20 human tumors transplanted and passaged in nude mice was associated with a massive induction of endogenous murine leukemia virus (MuLV). Separation and growth of these viruses on various substrates indicated that both ecotropic and xenotropic MuLV were present in the induced mixture. Tryptic peptide fingerprints of the p30 and gp70 structural elements of the viruses indicated that all of the known endogenous muLVs of BALB/c mice were present in the mixture. In addition, a new xenotropic MuLV was identified. The human tumor that induced the viruses was an oat cell carcinoma. The oat cell carcinoma possibly produced a specific hormone or factor that acts as a potent inducer of endogenous type C retroviruses.