Mitoxantrone has demonstrated therapeutic efficacy in the regional treatment of intraperitoneal malignancies. However, severe local toxicity was dose limiting. Consequently, a new injectable sustained delivery formulation of mitoxantrone has been developed: the drug was incorporated (8.2 w/w%) in highly hydrophilic albumin microspheres. In vitro drug release profile was modified by matrix crosslinking extent. The extractable amount of residual crosslinking agent (glutaraldehyde) in the microspheres was lower than 6 ppm. Mitoxantrone concentration in peritoneal fluid and plasma was determined up to 72 h after intraperitoneal administration of 30, 60 and 120 mg mitoxantrone per m2 body surface area as solution and in the form of a dispersion containing mitoxantrone-loaded microspheres to rats. Data analysis revealed sustained release of mitoxantrone from microspheres into peritoneal fluid in all dosage groups. The initial high drug levels in peritoneal fluid and plasma observed after application of mitoxantrone in the solution form were prevented by administration of the drug incorporated in microspheres. However, tumoricidal drug levels in peritoneal fluid were maintained over a comparable time span. In addition, preliminary toxicity data suggest a superior local tolerability of mitoxantrone-loaded microspheres. The dose of intraperitoneally administered mitoxantrone might be increased from 30 to 60 mg per m2 body surface area using the slow release formulation. In conclusion, the described microsphere drug delivery system for mitoxantrone might overcome dose-limiting drug toxicity.