Seventy-nine patients that had persistent or recurrent secretory otitis media for over 3 months and were scheduled for myringotomy and/or adenoidectomy received a single dose of cefaclor (20 or 40 mg/kg) orally 0.5-4 hours before surgery. Cefaclor concentrations were determined in serum, middle ear fluid and/or blood-free adenoid tissue homogenate using microbiological methods. Concentrations in adenoid homogenate ran parallel to those in serum, indicating a rapid penetration to adenoid tissue. Penetration to middle ear fluid was delayed in comparison. In the 51 patients receiving 20 mg/kg, peak levels occurred after 0.5 hour in serum as well as in adenoid homogenate and middle ear fluid with mean values of 29, 7.5 and 5.1 mg/l, respectively. Concentrations in the middle ear fluid were greater than 2.0 mg/l from 0.5 to 3 hours after this dose. In the 28 patients receiving 40 mg/kg, concentrations levelled after 0.5 hour in serum and in adenoid homogenate, with mean values of 21-33 mg/l and 6.3-12 mg/l, respectively. Concentrations in the middle ear fluid reached 7.8 mg/l 2 hours after this dose. A dose of 20 mg/kg appears to be suitable for treatment of respiratory tract infections on a b.i.d. basis.