Several amino acid analogs were tested for their ability to induce type C virus from a cloned Balb/c mouse cell line transformed by Kirsten sarcoma virus. O-Methylthreonine and hydroxynorvaline, analogs of isoleucine and threonine, respectively, were found to induce type C virus with the host range of Balb:virus-2. Type C virus was also activated by lysine deprivation. When either O-methylthreonine or hydroxynorvaline was used in conjunction with the arginine analog canavanine, the induction equalled or surpassed the maximum induction by cycloheximide. Using synchronized cells, induction of virus by canavanine was found to be cell-cycle dependent. Depriving cells of certain single amino acids alone or in combination with specific analogs markedly reduced protein synthesis without inducing virus. The results suggest that a general reduction in protein synthesis cannot account for the induction and imply that the inducing analogs may affect the functionality or degradation of proteins involved in the regulation of virus expression.