The N-tropic strain of Friend virus (FV) complex includes two different infectious entities: defective SFFV and helper virus, the latter of which is responsible for the N-tropism of the complex. The N-tropic helper, highly infectious in homozygous Fv-1ⁿ mice, is poorly infectious in Fv-1^b mice. B/T-L virus, a leukemogenic virus free of SFFV, includes a B-tropic helper virus which is much more infectious in Fv-1^b than in in Fv-1ⁿ mice. N-tropic FV complex to which B/T-L virus has been added (i.e., SFFV with its associated N-tropic helper, plus B-tropic helper) induced FV disease in young Fv-1^b mice and gave rise to a strain of virus which could be passaged serially through Fv-1^b hosts. Analysis of the host range properties of the new virus strain indicated that the N-tropic helper component of FV complex had been rapidly eliminated during the first few passage generations of the virus, leaving a B-tropic pseudotype of SFFV, i.e., SFFV associated with B-tropic helper virus. Susceptibility to this B-tropic pseudotype was influenced in the expected manner by the Fv-1 and Fv-2 genes, but host range studies indicated that one or more previously unidentified genes also influenced the host response to this virus. Serial forced passage of the B-tropic FV through Fv-1 -restrictive hosts resulted in the recovery of an NB-tropic variant of the pseudotype.