For continuous release of cytokines for local application at tumor sites, we developed depot preparations consisting of a carrier substance (ethylene vinyl acetate copolymer) and either natural interleukin-2 (nIL-2) at 5 x 10(5) BRMP U or natural interferon-alpha (nIFN alpha) at 3 x 10(5) IU. We transplanted human tumors in nude mice and placed depot preparations next to the tumors to confirm the in vivo long-term biological activity of the depots according to the ability of IL-2 to inhibit growth of the tumors. Five different human tumors were used. Mice with each tumor were divided into groups of six to eight receiving different treatments: control (no treatment), human serum albumin, but no cytokine, IL-2 depot, IFN alpha depot and IFN alpha plus IL-2 depot. After proven in vivo tumor growth, these depot preparations were implanted subcutaneously directly at the tumor sites. Tumor growth was significantly reduced by IL-2 and IFN alpha plus IL-2 for 3 weeks after depot implantation. Complete tumor remission was not achieved. IFN alpha alone did not influence tumor growth. Our data show that the nude mouse is a valuable model for testing biological activity and release time of IL-2 depot preparations in vivo.