Abstract Chick embryo cell supernates or sera of young line 100 chickens, which were susceptible to avian leukosis-sarcoma viruses of subgroup E, contained large amounts of an avian leukosis virus. This virus had biological, chemical and physical properties identical with RAV-O, an endogenous subgroup E virus of the chicken. Line 100 cells homozygous for a recessive gene for cellular resistance ( tvb r ) did not produce significant amounts of RAV-O naturally, but did produce this virus on cocultivation with cells which were susceptible to subgroup E. Crosses of line 100 or related inbred line 7 with other lines showed that the dominant I e gene, which inhibits susceptibility to subgroup E, did not block RAV-O production, but merely restricted its spread from cell to cell. These data suggest that lines 7 and 100 carry dominant genes for RAV-O production which act independently from the known genes controlling resistance to subgroup E viruses. The phenotype associated with RAV-O production is called V +.