We have developed a novel sustained release formulation, minipellet, which is applicable to various kinds of biologically active proteins. In this study, dosage form design of interferon (IFN) minipellet was investigated. First, in order to select suitable carrier material, matrix type formulations were prepared with natural biodegradable polymers, human serum albumin (HSA), gelatin and atelocollagen, and the release profiles of IFN from these polymers were compared. IFN was released slowly from the sample made of atelocollagen but rapidly from the samples made of HSA and gelatin. The release of IFN from the atelocollagen films, prepared by drying the atelocollagen solution, varied with the atelocollagen concentration before drying. This suggested that IFN release was controlled by the density of atelocollagen matrix. So, in order to obtain the higher matrix density of atelocollagen, we newly designed a cylindical dosage formulation prepared by extrusion and air-drying of an atelocollagen solution with high concentration and named minipellet. IFN was constantly released from minipellet in vitro. Pharmacokinetic studies in mice showed that the serum IFN concentration was maintained for a long time with minipellet. To determin the optimal minipellet composition, IFN minipellets containing different amount of HSA were prepared and IFN release profile from them in dogs was evaluated, The IFN concentration increased gradually after the administration of IFN minipellet containing 30% (w/w) HSA, reached Cmax after 24h, and decreased gradually thereafter with a detectable level for 10 days. The IFN minipellet is expected to offer a significant advantage in IFN therapy because the attenuated peak IFN concentrations in serum may reduce the side effects and the sustained release may reduce the frequency of administration.