ABSTRACT Prolactin levels were found in human plasma during physiological and pathological lactation. The biological activity was measured by means of a modified method of the local pigeon crop test by Grosvenor & Turner (1958). The biological activity found in these cases could be neutralized by an antihuman prolactin. This antibody which was not purified by incubation against the serum of hypophysectomized women was received by immunization of rabbits with a prolactin extract of human pituitaries without any albumin contaminations. These experiments may show that the biological prolactin activity containing in the hypophyseal extracts is not an artefact of the extraction procedure. The high plasma-prolactin concentration in pathological lactation without any acromegalic symptoms could be a further argument for the hypothesis that also in man prolactin and growth hormone could be isolated proteohormones.