BT penicillin (n-butylthiomethyl penicillin) prepd. by the addition of precursors to the media upon which Penicillium notatum was grown was compared to benzyl penicillin in 11 patients. Comparison of penicillin concns. in the plasma and cerebrospinal fluid was made following the intraven. injn. of 500,000 U. of each of the penicillins. Following penicillin G, the avg. plasma concn. at 5 min. was 49.36 U. and at 120 min. it was 1.02 U./ml. Following BT penicillin, the avg. plasma concn. was 32.51 U. at 5 min. and at 120 min. 0.38 U./ml. Penicillin assay of the CSF 120 min. after the intraven. injn. of 500,000 U. showed that the avg, concn. after penicillin G was 0.03 U./ml. and after BT it was 0.01 U./ml. Although only 11 patients were studied, each patient was studied after the injn. of both penicillins; BT penicillin probably did not diffuse into the CSF as readily as penicillin G.