Cell lines have been established in culture from Friend virus-induced tumors of BALB/c (H-2d) and congenic BALB/c-H-2b (BALB.B) origin. Spleens from virus-infected hosts in the terminal stages of erythroleukemic disease provided tissues for the establishment of subcutaneously transplantable tumors of both strains. Subsequently cells of these tumors were introduced into culture and passed serially. Complete, infectious Friend virus (FV) has been routinely recovered from culture supernates of BALB.B tumor cells (HFL/b) throughout its 2-yr passage history. However, after only a few transfer generations in culture BALB/c tumor cells (HFL/d) became nonproducers of virus detectable in either the spleen focus assay in vivo or the XC assay in vitro. Nonproducer HFL/d cells possessed the complete genomes of the components of the FV complex, since FV could be recovered from them either by cocultivation with helper virus-infected syngeneic embryo fibroblasts or by serial passage in the ascitic form in normal, syngeneic adult hosts.