1.2 mega U Penicillin V potassium was given to 20 patients 2 hours (group 1) and another 19 patients 3 hours (group 2) before tonsillectomy. After the operation biological determinations of the levels of active principle were made from tonsillar tissue and serum with statistical evaluation of the results (Spearman's rank correlation). There was a significant connection between the serum and tonsillar tissue concentrations (in children and adults) in both groups. The level of the active principle in group 2 was only slightly lower than in group 1. Many of the micro-organisms responsible for tonsillitis can be influenced by the concentrations found here, so that application of Penicillin V can often be expected to have a good therapeutic effect.