Navelbine (NVB) is a new semi-synthetic Vinca alkaloid selected on the basis of its affinity for tubulin. NVB inhibits the polymerisation of tubulin and it has significant antitumor activity on P388 and L1210 leukemias and some other experimental tumors. In the present study, 20 patients (9 carcinomas, 10 lymphomas and 1 blastic crisis of chronic myeloid leukemia) received a median of 4 weekly i.v. doses of NVB. Two patients at least received each dose level: 3.6 mg/m2 (1/10 of the LD10 dose/kg in BDF1 mice), 7.2, 12, 18, 32.4, 35 and 43 mg/m2 per week. A total of 89 doses were administered. All patients had been first heavily pretreated and 17 of them had received a Vinca alkaloid. Leukopenia (neutropenia) was the dose-limiting toxicity. There was no thrombocytopenia. Leukopenia was dose-related and first seen at 32.4 mg/m2 per week. The maximal tolerated dose appears to be about 43 mg/m2. At that dose, 2 out of 3 patients developed severe leukopenia and neutropenia. One localized allergic reaction, one case of transient hepatic dysfunction, and 2 reversible peripheral neuropathies were seen. Pharmacokinetics, studied with a radioimmunoassay (RIA) method, suggested an elimination half-life of 30 h and a plasma clearance of 75 l/h. Four patients with Hodgkin's disease and two patients with non-Hodgkin's lymphoma, all of them refractory to vincristine (VCR) and/or vinblastine (VBL), showed minor responses lasting 2-8 weeks. They had received between 4 and 12 doses of 30 and 43 mg/m2. We recommend for phase 11 trials the dose of 40 mg/m2 per week.