SummaryConcentrations of erythromycin in middle ear effusions and in plasma were measured at various periods of time in a total of 65 patients, with secretory otitis media requiring myringotomy, who had been treated with a standard dosage for up to 10 days. After an initial slow penetration into the effusion, the mean concentrations of erythromycin were equal in plasma and secretion at the steady state level, although higher concentrations in secretion than in plasma were measured in more than half the patients. Erythromycin remained at higher concentrations and for a longer time in middle ear secretions than in plasma when administration was stopped.