The C-type particles produced by the A9 and A9HT sublines of mouse L cells were shown to infect C3H (N type), but not C57BL (B type), mouse embryo fibroblasts. Infection was indicated by distinct single giant cell formation in the XC monolayer used to overlay the mouse embryo fibroblasts. On the basis of these results it was concluded that the L cell virus is N tropic. A9 and A9HT cells were fused to various mouse cells derived from tumors and normal tissues. The ability to produce the Moloney-type surface antigen and to release infectious virus was introduced by the A9 component into the hybrid cell. Virus production, measured by antigen induction on JLS-V9 cells, was suppressed in those hybrids in which the partner cell had a genotype determining low infectibility with that particular virus (B-type cell). It thus appears that the major genetic locus affecting resistance to infection with leukemia viruses, the Fv-1 locus, regulates infectious virus production in somatic cell hybrids also. The same genetic locus did not seem to govern the expression of all virus-related functions, for the virus-determined membrane antigen was demonstrated in many of the N x B-type hybrids in which production of infectious virus was suppressed
