Avian leukosis virus (ALV)-induced osteopetrosis is characterized by the presence of unintegrated viral DNA is diseased bone. To learn more about this phenomenon, ALVs with high (Br21) and low (RAV-1) osteopetrotic potential have been compared for the ability to establish superinfection resistance in cultures of calvarial-derived osteoblasts. Both viruses could establish interference in osteoblast cultures that were maintained as growing cells. This interference was maintained when cultures were induced to differentiate. These results suggest that the unintegrated viral DNA in osteopetrotic bone is not due to the inability of osteopetrosis-inducing viruses to establish superinfection resistance in osteoblasts.