[en] The molecular structure of murine retroviruses expressed in spontaneous and radiation-induced bone tumours was studied. These viruses induce osteomas, lymphomas and osteopetroses in mice of the NMRI strain. RNase T₁ fingerprint analysis indicates the presence of mixed virus populations in the tumours, with major components showing close relationship to Akv MuLV. Cloned viruses, closely related to Akv MuLV, have the same oncogenic properties as the original mixtures. In its nucleotide sequence of the repeat segments of the transcriptional enhancer in the LTR, one cloned virus analysed was distinct from Akv MuLV, but closely related to a spontaneous bone tumour virus isolate, FBJ MuLV. (author)