An analytical method for the determination of mitoxantrone in bone marrow was developed using high-performance liquid chromatography with electrochemical detection. The extraction procedure was optimized by investigating several factors which potentially could influence the recovery of mitoxantrone from bone marrow cells. The mean recovery of mitoxantrone from rat bone marrow was found to be 81.7% with a coefficient of variation 3.8%. High-performance liquid chromatography was carried out to quantitate mitoxantrone using ametantrone as internal standard. The detection limit of our analytical method amounts to 100 pg on-column, corresponding to 1 ng/ml of cell suspension containing 2 · 107 cells and a day-to-day variation of maximally 8%. Storage of bone marrow samples, containing mitoxantrone, for one to fourteen days resulted in a mean recovery of 94%, as compared to freshly analysed samples. Subsequently we studied the pharmacokinetics of mitoxantrone in rat bone marrow. It appeared that after an intravenous bolus injection of mitoxantrone (2.5 mg/kg) in rats, the drug accumulated in the femoral bone marrow for about four days, and thereafter gradually declined.