Eight cases of malignant tumors of the head and neck were treated with NK 631 on a dosage schedule of 10 mg at a time 3 times weekly, by intravenous one-shot injection or intravenous drip infusion, to observe its therapeutic effects and adverse reactions. The treatment was assessed markedly effective in 3, moderately effective in 1 and ineffective in 4 of them. The treatment was also assessed moderately or markedly effective in 3 and ineffective in 2 out of squamous cell carcinoma cases. Hematologic findings, serum electrolytes and enzymologic findings were normal, but the pulmonary function examinations revealed a tendency for PaO2 to decrease slightly. In 1 case where frequent cough was observed, the cough was mitigated on withdrawal of the treatment. The adverse reactions that evolved included fever, alopecia, eruptions, nausea and vomiting, and pigmentation of the nail. To summarize these findings, the authors were impressed with NK 631 and that the agent would exert an excellent antitumor effect, compared with bleomycin, and that its effect would evolve at the early stage of its treatment. Fixed drug eruption was observed as an adverse reaction of this drug in 1 case; however, the adverse reactions of this bleomycin analog appear similar to those of its parent compound, bleomycin.