B-cell and T-cell human leukemic cell lines, BALL-1 and TALL-1, were transplanted into newborn hamsters immunosuppressed with rabbit anti-hamster lymphocyte serum. The growth inhibitory effect of interferon (IF) was tested by intraperitoneal administration in these tumor-bearing animals. The following results were obtained: (1) TALL-1 transplanted intraperitoneally was inhibited much more than BALL-1 transplanted subcutaneously. The life span of hamsters transplanted with TALL-1 was also prolonged. These findings coincided with the results of in vitro sensitivity of these two cell lines to IF. (2) The antiproliferative effect of a combination of cyclophosphamide and IF to BALL-1 was greatly enhanced compared with when either agent was used alone. (3) The combined use of L-asparaginase and IF was similarly more inhibitory to TALL-1 than either agent alone. It seems that the growth of residual tumor cells after chemotherapy can be effectively inhibited by IF. In using IF for cancer chemotherapy, therefore, it would be advisable to combine chemotherapeutic agents with IF.