Levels of roxithromycin in serum and tissue were investigated in 29 subjects undergoing tonsillectomy. A total of 13 subjects received a single oral dose of 300 mg, and 16 received four oral doses 12 h apart as follows: a 300-mg loading dose followed by three 150-mg doses. Measurable levels of roxithromycin were present in tonsil samples of 11 of 13 subjects in the first group. The mean levels in tonsils and serum were 0.8 microgram/g and 6.7 micrograms/ml, resulting in a mean tissue/serum ratio of 0.16. In the multiple-dose group, roxithromycin was found in 14 of 16 subjects at mean levels in tonsils and serum of 1.6 micrograms/g and 8.7 micrograms/ml, and the tissue/serum ratio was 0.23.