After oral administration of 300 mg Sobelin, the efficacy and pharmacological kinetics of clindamycin (CAS 18323-44-9) were analysed in 35 patients with recurrent tonsillitis. Minimal inhibitor concentration (MIC) and minimal bactericidal concentration (MBC) have been evaluated for 120 strains. MIC and MBC for 91.7% of the strains amounted to 0.25 micrograms/ml. Concentrations of antibiotics in the serum were higher than 0.25 micrograms/ml after 12 h, i.e. higher than the minimal inhibitory concentration. The established free and efficient portion of clindamycin in the serum which is not bound to plasma proteins shows that a clear bacteriostatic effect is achieved up to 6 h after application of the antibiotic. The concentration in the tissue of the tonsils was 0.6-0.8 micrograms/g after 7-9 h indicating that sufficient MIC values were achieved. The free portion of the antibiotic in the tissues after 9 h also reached the MIC of germs. Hence clindamycin is considered to be an efficient antibiotic in the management of acute exacerbation of recurrent tonsillitis. Due to serum- and tissue levels of the tonsils administration of 300 mg of the test drug is indicated three times daily.