To examine whether the resistance allele of the Fv-4 gene (the Fv-4r gene) is a dominant inhibitory-product-encoding gene which an be used to prevent the development of murine AIDS (MAIDS), bone marrow cells from BALB/c-Fv-4wr mice were transplanted into BALB/c mice and C57BL/6 mice infected with MAIDS virus. Almost all of the virus-infected BALB/c and C57BL/6 mice developed MAIDS within 4 months and died 2 or 3 months later. However, when the virus-infected mice were subjected to cobalt irradiation and then given an intravenous injection of 10(7) BALB/c-Fv-4wr mouse bone marrow cells, the recipient mice survived much longer than the untreated mice, which suggests that the Fv-4 gene is a dominant inhibitory gene that is potentially useful in gene therapy of MAIDS.