A radioimmunoassay for primate prolactin has been developed, with [(131)I] monkey prolactin, and antibodies to monkey or human prolactin. The assay is specific for prolactin; human growth hormone, and human and monkey placental lactogen show no significant crossreaction. The assay is sensitive enough to measure prolactin concentrations in the sera of most humans studied. The concentration of prolactin in the serum of normal children and adults of either sex was usually below 30 ng/ml, while very high concentrations (up to 500 ng/ml) were observed in newborn infants. The serum prolactin concentration during the menstrual cycle showed no definite increase in the luteal phase. Of 24 patients with galactorrhea, 20 had prolactin concentrations above 30 ng/ml; the highest value observed was 1500 ng/ml. In contrast, 12 of 13 patients with acromegaly had concentrations within the normal range. During pregnancy, the concentration of prolactin in serum rose progressively from an average of 30 ng/ml in the first trimester to 200 ng/ml at term. Postpartum, prolactin concentrations fell to normal levels after 1-2 weeks. Suckling was a potent stimulus to prolactin release, increasing its concentration in serum some 10- to 20-fold.
