The progestagen concentration of milk was measured in cows, at various times of the reproductive cycle by a competitive protein binding procedure and by radioimmunoassay. The apparent progesterone concentration was found to be significantly greater in milk (P < 0-05) than in plasma, probably due to the presence of an unidentified metabolite(s). A simple, rapid radioimmunoassay was developed to measure directly the progestagen concentration in milk. A mean concentration of about 1 ng/ml at oestrus (or mating) increased, 20-24 days later, to 3-4 ng/ml in cows subsequently found to be non-pregnant and to about 7 ng/ml in pregnant animals. The possible application of this analytical procedure as the basis of a method of pregnancy diagnosis is discussed.

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