The survival of bone-marrow granulocyte-macrophage colony-forming units (CFU-GM), an alkylating-agent-sensitive normal tissue, was assessed in mice bearing the EMT-6 parental tumor or the in vivo resistant EMT-6/CDDP, EMT-6/CTX, EMT-6/Thio, and EMT-6/Carbo tumors. The survival pattern of the bone-marrow CFU-GM recapitulated the survival of the tumor cells, mimicking the development of resistance and reversion to sensitivity upon removal of the selection pressure for each of the four alkylating agents. When the EMT-6 parental tumor was implanted in the opposite hind limb of animals bearing the EMT-6/CDDP or EMT-6/CTX tumor, the survival of the parental tumor cells after treatment of the animals with the appropriate antitumor alkylating agent was enhanced. The EMT-6/CDDP tumor was cross-resistant to CTX and high-dose L-PAM, whereas the EMT-6/CTX tumor was somewhat resistant to CDDP and markedly sensitive to VP-16. In each case, the survival pattern of the bone-marrow CFU-GM reflected the survival of the tumor cells. These results indicate that the presence of an alkylating-agent-resistant tumor in an animal can affect the drug response of tissues distal to that tumor.