A group of 41 patients with various malignant diseases were treated with vinblastine and a second group of 27 patients were treated with vincristine. In some cases, radiation therapy was included with drug therapy. The duration of treatment averaged 7.5 weeks with vinblastine and 6.8 weeks with vincristine. Athough dosage was adjusted to white blood count level, leukopenia developed in 80% of patients treated. Objective improvement was noted in 29.3% of the vinblastine group and in 37% of the vincristine group. Neurological side effects were noted only in patients treated with vincristine.