The disc assay of the International Dairy Federation for determination of penicillin in milk has been modified in the following steps: preparation of seed broth, seeded plates, and standard solutions. The modified assay is sensitive and relatively rapid, requiring a 4-h incubation at 55 C. Approximately 0.004 unit of penicillin/ml of milk can be detected on plates stored for 7 days; with freshly prepared plates and standard solutions, as little as 0.002 unit of penicillin/ml can be detected.

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