The effect of erythromycin stearate at a dose of 1 g daily on theophylline disposition was studied in six healthy adult males. The mean value of 5.46 h for the theophylline half-life after 48 h pretreatment with erythromycin stearate was not significantly different from the control value of 5.24 h. The half-life value of 8.48 h after a 1-week course of the antibiotic, however, represented a significant increase (p less than 0.005). Theophylline disposition 1 week after the last dose of the antibiotic was not significantly different from the control. It is suggested that therapeutic doses of erythromycin stearate for a week or more could impair the disposition of concurrently administered theophylline. The impairment however, is reversible, being absent a week after the last antibiotic dose.