The concentration of progesterone in milk and blood plasma was determined by radioimmunoassay in 56 pregnant cows. Samples were from eight cows on each of the following days of pregnancy: 30, 60, 90, 120, 150, 180, and 210. Validity of the assay was indicated by parallelism among unknowns, whether milk or plasma, and between unknowns and the standard. Progesterone was higher (P < 0.005) in milk (21.2 ng/ml) than in blood plasma (5.3 ng/ml) averaged over all days of pregnancy. The difference among days and the interaction of day and type of fluid (milk vs plasma) were not significant.
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Ginther et al. (1974) studied this question.