Nineteen patients with multiple myeloma resistant to standard alkylating agent therapy or to the VAD regimen received carboplatin at a planned daily dose of 100 mg/M2 on four successive days. Two patients erroneously received a four-fold higher drug dose resulting in bone marrow aplasia and death without antitumor effect in one patient with post-mortem examination. No anti-tumor effect was observed among 15 patients evaluable for response (two lacked follow-up examination of tumor markers). Major toxicities were hematologic and included grade > or = III, leukopenia in 9, thrombocytopenia in 6 and anemia in 3 of the 17 evaluable patients. Their median survival was 9 months. These results indicate that carboplatin is inactive in refractory multiple myeloma.