An early phase II clinical study of RP56976 (docetaxel), a new anticancer agent of plant origin, was conducted in patients with breast cancer at 20 Japanese collaborative institutions. Docetaxel was administered at two or more doses of 60 mg/m2 by intravenous infusion with dose-free intervals of 3-4 weeks, and the efficacy and safety was evaluated. Of the 51 patients enrolled, 50 patients completed the scheduled course of treatment. Two patients showed a complete response (CR) and 19 showed a partial response (PR) with a response rate of 42.0%. The response rates based on the efficacy for metastatic lesions in soft tissue, liver and lung, were 46.2% (18/39), 37.5% (3/8), and 38.5% (5/13), respectively. Of the 50 patients who completed the study, 48 patients had previously been treated for the present malignancy. Forty-seven patients had previously been treated with chemotherapy and showed a response rate of 40.4% (19/47). The response rate in those who had received chemotherapy composed of anthracyclines and other agents was 44.1% (15/34). Grade 3 or more severe leukopenia and neutropenia developed in 43 patients (84.3%) and 48 patients (94.1%), respectively. Other adverse reactions which occurred in a Grade 3 or more severe form included nausea/vomiting (1 patient), anorexia (5 patients), diarrhea (4 patients), fatigue (2 patients), and alopecia (20 patients). Except for alopecia, most adverse reactions were generally transient and reversible without any specific treatment.