Two nonvirus-producing subclones of the murine sarcoma virus-transformed rat nonproducer cell, K-NRK Cl 32, which have partially regained the property of contact inhibition of cell growth, have been isolated. Both subclones, unlike the trans formed and untransformed parental cell lines, K-NRK Cl 32 and NRK, are resistant to exogenous infection by type-C RNA tumor viruses from several species; this block can be overcome at high virus input multiplicity. Iododeoxyuridine induction of endogenous rat type-C virus proceeds normally in spite of the block to exogenous viral infection.