25,000 units of pure crystalline penicillins G, X, and K, dissolved in normal saline at a conc, of 5,000 U./ml., were injd. individually on different days into each of the same 4 subjects. Urine samples were assayed by the cylinder-plate method against Staphylococcus aureus 209 P. and blood levels were detd. by the method of Heilrnan against her strain of hemolytic streptococcus. A penicillin G standard was used. The duration of blood levels of at least 0.03 U./ml. for each was: penicillin G, 2-2.5 hrs.; penicillin 128, 1-1.25 hrs., K, 0.5-0.75 hrs., and X, 4-4.5 hrs. In the first 2 hrs. the penicillins were excreted in the following percentages: G, 83; 128, 58; K, 28; and X, 78. Since very little penicillin is excreted after the 2d hr., K is very rapidly destroyed, and its therapeutic value must be very small. G and X are stable and their excretion represents the limiting factor in maintaining blood levels. X is more active against at least 1 strain of a hemolytic streptococcus and its use is indicated for an infection by any organism of similar sensitivity.