Summary 1. As determined by the customary cup test procedure it is not possibl obtain complete recoveries of penicillin G and K when added to plasma andtissues. A considerable error involved in the assay penicillin K in the presence of a high concentration of pld ma Such low recoveries have been shonn not to be due to destruction of penicillin K 2. For a given dose. the plasma level of penicillin K 5 minutes after intravenous injection is less than one that with penicillin G The rate of disappearance from a given plasma concentration, however, is not greatly different for the 2 penicillins. 3. The apparent volume of distribution of G is 20.2% of the body weight, whereas with K it is 57.1%. Both of the penicillins are localized in lung and liver to erne extent. Penicillin K is localized to a considerably greater extent in the liver than is G. 4. is determivd by dialysis experiments penicillin K is atlsorhrti by some component or' dog plasma to a much greater extent than is penicillin G.