When Sarcina lutea is used as test organism in a cup-plate assay, the dose-response curves for the assay of penicillin in sera of humans and rabbits are linear with the square of the zone diameter and log concentration of penicillin and parallel to the dose-response curve for penicillin in phosphate buffer. Suitable conditions for such assays are described, using either large square assay-plates and fish-spine beads or conventional Petri dishes and cylinders. The assay using large plates and fish-spine beads is much more accurate and economical of effort and serum. An assay method suitable for inclusion in the blood-level duration test for oily injection of procaine penicillin (International Pharmacopoeia) is described. The method makes it possible for fiducial limits to be readily computed for the potencies obtained.
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