Eleven asthmatic men were treated with the antimetabolites thioguanosine or 6-mercaptopurine. Eight completed 12 weeks of treatment. Reversible drug toxicity occurred in 2 patients. Four patients with severe chronic asthma, chronic bronchitis, and continuous wheezing were not improved. Five patients with intermittent asthma of recent onset improved and had no severe attacks during therapy. Two of them showed consistently improved air flow rates and blood oxygenation, though some obstruction to air flow persisted. The cytology and types of mucus in stained sections of sputum helped to classify patients. Immunosuppressive therapy may prove to be useful in selected patients who have asthma but do not have chronic bronchitis.