The antiproliferative potency of mitoxantrone (MITOX) has predominantly been established for epithelial and hematologic neoplasias. In this study the effectiveness of MITOX was investigated in vitro for 6 sarcomatous human cell lines derived from 2 synovial sarcomas, a malignant schwannoma, a malignant histiocytoma, a leiomyosarcoma, and a chondrosarcoma. The examination was performed using a proliferation assay with monolayer cell cultures. The effect of MITOX was compared with that of adriamycin (ADR) and cisplatin (CDDP). For each drug at least 3 concentrations were tested which covered the therapeutically achievable range, i.e., for MITOX 0.2-0.002 micrograms/ml, for ADR 0.5-0.005 micrograms/ml, and for CDDP 5.0-0.05 micrograms/ml. Test incubations were performed for 3 days. Antiproliferative potency of the cytostatic drugs was assessed by counting the number of cells at the start and the end of the test period with and without drug addition. Furthermore the dose inhibiting cell growth to 50% of controls (ID50) was determined for MITOX. For comparison 4 cell lines from carcinomatous lesions were included in the study. MITOX inhibited proliferation rates of 4 sarcomatous tumor cell lines more intensively than ADR, and was less effective in 2 cell lines. However, these differences were not significant. In all mesenchymal cell lines tested the antiproliferative potency of MITOX was more pronounced than that of CDDP. In carcinomatous cell lines the MITOX-induced growth inhibition was similar to that found in response to administration of ADR and CDDP confirming the described effect on epithelial tumors. The study suggests that MITOX possesses a growth inhibitory potency for malignant soft tissue tumors in vitro. From these data it may be worthwhile to initiate clinical trials testing the treatment of sarcomatous lesions with MITOX.