A sensitive and specific assay for human prolactin has been developed using human prolactin and antiserum distributed by the United States National Pituitary Agency. Plasma prolactin concentrations in control subjects ranged from 0-20 ng/ml. No sex difference in prolactin concentration was observed. A brisk increase in plasma prolactin levels occurred in normal subjects during the administration of chlorpromazine and thyroid stimulating hormone releasing factor (TRH). These stimulatory tests of prolactin release should therefore be useful in the assessment of hypothalamic pituitary function. Basal plasma prolactin values were raised in most patients who were being treated with phenothiazines and were helpful diagnostically in patients with amenorrhoea, galactorrhoea, hypogonadism, cranio-pharyngioma, "non-functioning" pituitary tumours and acromegaly. In many of these disorders a significant reduction in the plasma prolactin concentration was observed following oral administration of bromocriptine. Plasma prolactin determinations should be useful in evaluating the response to medical treatment or pituitary ablation.