A non-leukemogmic strain of Friend leukemia virus ( F L V ) propagated in vitro interfered with infection by murine sarcoma virus (Moloney) ( M S V ) and acted as a helper of defective MSV. This suggests that the non-leukemogenic strain of FLV is infectious. After prolonged propagation in vitro, the Ieukemogenicity of a highly leukemogenic strain of FL V gradually decreased, while the infectivity assayed by iriterference with MS V infection remained unchanged.