Simian sarcoma virus, type 1 (SSV-1)-transformed non-producer cell lines were established by infection of normal marmoset fibroblast cells (HF) with limiting dilutions of SSV-1. Four focus-derived cell lines were identified as non-producers by assay of culture fluids for focus-forming activity and by the mixed culture cytopathogenicity test with XC cells. Further studies failed to detect production of type-C virus by 3H-uridine labelling, reverse transcriptase assay or electron microscopy. The non-producer cell lines, designated HF/SSV-NPI, IV, V, and VI, appeared morphologically transformed and cloned in soft agar with the same efficiency as HF/SSV-1 virus-producing transformed cells. Expression of either cytoplasmic or cell surface virus-related antigens was not detected by immunofluorescence or serum cytotoxicity tests. The presence of sarcoma genome in the transformed non-producer cell lines was demonstrated by rescue of focus-forming activity following superinfection with non-transforming helper virus or by cocultivation with helper-virus-producing cell lines. The SSV-1-transformed non-producer primate cells provide a useful tool for future studies.