Avian leukosis virus (ALV) is known to cause several neoplastic conditions in chickens, such as B-cell lymphomas, myelocytomas, erythroblastosis, and other types of neoplasia including osteopetrosis. We describe herein the identification of unique ALV-related proviral DNA sequences in an archived chicken bone affected with osteopetrosis. The osteopetrotic bone was obtained from an affected 46-week-old brown layer during an outbreak of osteopetrosis in Costa Rica in 1986. Analysis of proviral DNA in the 23-year-old osteopetrotic bone revealed unique exogenous ALV-related sequences that were named CR-1986 (Costa Rica, 1986). The 5′ and 3′ long terminal repeats (LTR) in the proviral DNA were identical to each other. The U3 regions in the LTRs were most similar to equivalent sequences in ALV-J, while U5 was identical to known endogenous ALV-E sequences. The predicted CR-1986 envelope protein was most similar to the envelope of myeloblastosis associated virus type 1 (MAV-1), although the percentage of amino acid sequence similarity to MAV-1 was low (90.4%). The variable and hypervariable regions of gp85 displayed several mutations compared to representative strains of ALV. The gp37 (transmembrane or TM) envelope protein showed three leucine to serine mutations that may represent important changes in the conformation of this protein, a finding that is currently being investigated. Several recombination events may have contributed to the emergence of CR-1986 because each analyzed segment was similar to a different ALV. CR-1986 may represent a unique ALV based on distinctive characteristics of its predicted envelope protein in comparison to previously reported ALVs.