Although most inbred strains of mice contain endogenous retroviral sequences, these sequences are usually not capable of producing infectious retroviruses. In some cases, the retroviral sequences are small fragments of viral genomes. In a few cases the sequences for complete retroviruses exist, but contain small defects which prevent the production of infectious virus. While the ability of reversions of these defects to produce retroviruses has been studied by site-directed mutagenesis and chemical-mediated mutagenesis, the presence of spontaneously occurring reversions has not been completely evaluated. We characterized an infectious ecotropic retrovirus spontaneously expressed in aged DBA/2 mice, which carries the complete but defective Emv-3. Although this endogenously produced retrovirus was related to the endogenous Emv-3 sequence, it had undergone recombination with another retroviral sequence to correct the defect which resided in the gag of Emv-3. Thus, recombination of endogenous ecotropic retroviral sequences may be a mechanism to produce infectious retroviruses in adult animals that contributes to pathologic disease.