Backcross mice assorting for the two ecotropic murine leukemia virus loci of SJL/J mice (Emv-9 and Emv-10) were tested for production of infectious leukemia viruses. It was found that tail tissue from mice carrying Emv-9 could not be induced to produce ecotropic MuLV whereas tail tissue from mice carrying Emv-10 were inducible. No mice showed detectable levels of spontaneous virus production. Preliminary restriction enzyme analyses indicated Emv-9 contains a deletion within the polymerase region.