This study describes an unusual type of virus seen when a feline leukemia virus (FeLV) pseudotype of murine sarcoma virus (MuSV) obtained by cocentrifugation procedures infected feline embryo cells (FEF) and two Crandell cat cell lines (CrFK1, CrFK2). When all three cell cultures were infected with MuSV-FeLV, only FEF and CrFK2 were transformed and only these showed normal and aberrant virus. The CrFK1 infected with MuSV-FeLV did not transform but did replicate normal type-C virus with a 50-A intermediate coat. The virus replicated in the two transformed lines showed three particles; a normal particle with a 50-A intermediate coat, a normal particle with a 100-A intermediate coat, and an aberrant particle with a 100-A intermediate coat.