IN 1947, Hooker and Forbes (1) described a bioassay for minute amounts of progesterone. The reaction involved changes in the stromal nuclei of the uterus of the castrated mouse and could detect the presence of 0.0002 μg. of progesterone in 0.0006 ml. of test fluid, i.e., 0.3 μg. per ml. In subsequent reports, Hooker and Forbes (2, 3, 4) indicated that the reaction was highly specific and that the hormone circulated in the blood, primarily in the free state, since less than 10 per cent was protein bound. Forbes (5) reported that frequently no bound progesterone could be detected during pregnancy in women and monkeys. Hence it would appear that, for free progesterone, determinations on untreated blood should suffice. This is in contrast with the estrogens, which are approximately 65 per cent protein bound (6, 7). Recently Salhanick, Olsen and Hisaw (8) reported that estradiol possessed the ability to block the action of progesterone on the stromal nuclei of the uterus of the castrated mouse. This could easily invalidate the assay and give erroneous results, particularly in tests which have been carried out on untreated blood samples containing both estrogen and progesterone.
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Zarrow et al. (1953) studied this question.