A normally, relatively sensitive P 388 developed resistant within few passages (P 388/Mitrox) by in vivo treatment with suboptimal doses (1 mg/kg i.v.) of mitoxantrone. This resistance remained stable over 50 generations without further drug treatment. Immunization with irradiated cells (30 Gy) 7 days before tumor challenge led to partial rejection, proving that there was a higher immunogenicity of the resistant line in comparison to the parenteral P 388 line. The P 388/Mitox showed cross-resistance towards doxorubicin, daunorubicin and vincristine. Cis-DDP and bleomycin had in the resistant line significantly better antineoplastic efficiacy than in the source P 388 and should be taken into consideration as second-line therapy following developmemt of clinical mitoxantrone resistance. Nifedipine, a calcium channel blcoker, and the immunosuppressive agent ciclosporin A were able to overcome resistance partially, but the mechansims are still unclear. The P388/Mitox can be considered as an interesting in vivo model for further research concerning resistance mechanisms and reversal of resistance.