Influence of Protein-Binding on the Interpretation of Penicillin Activity In vivo.*

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Abstract
Crystalline penicillins X, G, F, dihydro F, and K, were each adm. intramusc. to healthy adults in single doses of 300,000 U. in normal saline. Blood levels of penicillin were then detd. by a tube dilution technique modified to give like concs. of serum in each tube. All of the above penicillins gave similar blood levels for as long as measurable quantities were present. The only exception was penicillin K which gave a slightly lower level at 15 min. Previous reports of higher and more prolonged serum concs. of penicillin X, and lower, more rapidly disappearing concs. of penicillin K, may be artefacts created by the antagonistic action of the different amts. of serum present in the tubes when the usual tube dilution techniques were used for assay.