The host range of the C particle produced by FLOPC-1 myeloma cells, FLOPC-1 murine myeloma-associated virus (FL-MuMAV), was assessed in terms of its ability to productively infect and/or induce new viral antigens in a variety of different cell lines. Production of C particle-like structures by cells exposed to FL-MuMAV) was determined by incorporation of [3H]uridine into particles with a density of 1.16 g/ml and/or measurement of RNA-dependent DNA polymerase activity in concentrated culture medium. to FL-MuMAV was capable of infecting NIH/3T3, normal rat kidney (NRK) cell, BALB/c 3T3, and the A31 clone of BALB/3T3 cells but not rabbit cell line, SIRC. Thus, it is an N, B-tropic murine virus as replication in NRK cells has been shown not to delineate a group of murine viruses with a separate host range (M. M. Lieber, C. J. Sherr, and G. J. Todero, 1974). Further neoantigens, reactive with anti-FL-MuMAV serum, were detected on infected cells. Production of the MuMAV-like particle and MuMAV-associated cell antigens in infected NIH/3T3 and NRK cells persisted for three subcultures. The limited production could not be explained by the lack of an RNA-dependent DNA polymerase or high-molecular-weight RNA as the particles possessed both of these properties. The particles produced by infected NIH/3T3 or NRK cells were antigenically and physicochemically similar to FL-MuMAV and not K-MuLV. The MuMAV-like particles produced by infected NIH/3T3 were capable of limited replication in NIH/3T3 and and BALB/3T3 cells, whereas NRK-MuMAV replicated for a limited period in NIH/3T3, NRK, and SIRC cells; i.e., they had a different host range than FL-MuMAV. The particles produced by infected BALB/3T3 and A31 cells had the same host range as FL-MuMAV. In certain situations, isotopically labeled particles with a density of 1.16 g/ml were produced which appeared to lack RNA-dependent DNA polymerase.