Summary 1. Twenty-three patients with pertussis were treated with lyophile hyperimmune human pertussis serum. 2. Twelve of the patients treated were in the 1-to 3-month age group. Seventy-five per cent of these infants had a good response. There were no deaths in this group. 3. Treatment early in the course of pertussis was effective. In addition, two patients treated in the eighth and ninth week of illness had a good response. 4. There were demonstrable circulating agglutinins following the administration of the serum. 5. The lymphocytosis of the blood was suppressed after the administration of the serum. 6. Eleven patients who had pneumonic involvement were treated with the serum in addition to sulfadiazine. 7. There was no extension or recurrence of the pulmonary involvement and no development of pneumonia after a full course of serum therapy.