In vivo attachment of beta-haemolytic streptococci Group A (GABHS) to the epithelial cells of the palatine tonsils was determined in 10 patients with current acute tonsillitis (AT-group) and in 10 healthy carriers of GABHS (C-group) according to a method described. Standard bacterial culture revealed growth of GABHS in both groups. In the AT-group, massive attachment of streptococcal chains to the tonsillar epithelium was noted. These chains proved positive for FITC-labelled antibodies against beta-haemolytic streptococci, and were only occasionally found in the C-group. The overall bacterial attachment was significantly greater in the AT-group than in the C-group (p less than 0.001). Phenoxymethylpenicillin treatment caused a dramatic decrease in the number of the attached GABHS bacteria within 24 h. Two weeks after completion of the penicillin treatment one individual in each group was still culture-positive for GABHS.