IMMUNOLOGICAL ALTERNATION OF LEUKEMIC CELLS In VivoAFTER TREATMENT WITH AN ANTITUMOR DRUG*

L1210 leukemia was transplanted serially in CDF1mice treated with 5-(3,3-dimethyl-1-triazeno)imidazole-4-carboxamide (DIC, NSC 45388). After four different lines (C lines) had been treated for several generations, a marked increase in survival time of untreated mice was observed. In contrast, mice treated with DIC or immunosuppressed with cyclophosphamide succumbed earlier with generalized leukemia. Furthermore, a C line showed unusually high sensitivity to chemotherapeutic treatment with 1,3 bis(2-chloroethyl)-1-nitrosourea. The data suggest that C lines acquired strong antigenicity for CDF1and DBA/2 hosts. DIC treatment may have selected highly antigenic variants or induced somatic mutations resulting in the appearance of strong new transplantation antigen(s).