Transplantable experimental tumor models were constructed to study the activities of recombinant human interleukin-15 (rhIL-15) against tumor recurrence and metastasis. The results showed that tumor nodule formation was retarded and tumor growth was inhibited in the subcutaneous tumor model of LA795 lung adenocarcinoma after treatment with rhIL-15, and the survival rate of T739 tumor-bearing mice treated with rhIL-15 was much higher than that of mice treated with either saline or with the same dose of rhIL-2. This indicats that rhIL-15 had better antitumor effect than rhIL-2 at the same dose level. In some rhIL-15 treated mice, the tumor cells inoculated subcutaneously were eradicated and there was no tumor formation even 138 days after tumor cell inoculation. The tumor-free mice were rechallenged with live tumor cells and no tumor reoccurred in the following two months in all of these mice, indicating that long-lasting antitumor systemic immunity developed. It was also shown that tumor recurrence and metastasis were inhibited markedly after treatment with rhIL-15, but not with the same dose of rhIL-2, in both subcutaneously and intravenously disseminated tumor models of LA795 lung adenocarcinoma. Simultaneously, the CTL and NK cell activities of the splenocytes obtained from tumor-bearing mice that had been treated with either rhIL-15 or rhIL-2 were both markedly enhanced. However, the enhancement of CTL and NK cell activities was more significant in rhIL-15 treated mice than that in rhIL-2 treated mice. This suggests that the anti-tumor effect of rhIL-15 in vivo was achieved by enhancing the CTL and NK cell activities in tumor immune response.