The VN-11 recombinant retroviruses, originally generated by co-transfection of the avian MH2 and AKRv viral genomes, were molecularly cloned from an infected mouse cell line named N11. The analysis of the proviral genome sequence from one of these recombinants showed a possible envAKR-mycMH2 fusion. Point mutations were also found in this envAKR-mycMH2 gene. The cloned viral genome was co-transfected with the neo gene into the psi 2 packaging cell line. Selected clones were shown to transcribe the viral genome and supernatants from these cultures, containing C-type particles, were used to infect primary cultures from mouse lymphoid tissues and brain. Proliferating macrophages and microglial cell clones were obtained, indicating that various types of cells of the mouse monocytic-macrophage lineage can be immortalized in spite of the absence of selection or special growth conditions.