The Endocrine Unit, Department of Medicine, Royal Postgraduate Medical School, Du Cane Road, London, W12 OHS (Received 16 December 1977) Radioimmunoassays for prolactin described previously require incubation periods of 3–8 days (Hwang, Guyda & Friesen, 1971; Sinha, Selby, Lewis & Vanderlaan, 1973; Aubert, Becker, Saxena & Raiti, 1974). In our studies on prolactin secretion by human pituitary adenomas (Mashiter, Adams, Beard & Holley, 1977; Trinder, Barratt, Sood, Holley, van Noorden, Joplin & Mashiter, 1977), we were anxious to obtain assay data more rapidly in order to maximize the experimental life of each preparation. This report describes a 24 h assay which retains the characteristics of our existing 7 day assay with which it is compared. Human prolactin VLS 1 was used as standard, human prolactin VLS 3 for iodination and rabbit anti-VLS 3 as antiserum, all generously donated by the National Pituitary Agency, NIAMDD, U.S.A. Labelled Na125I (IMS 30) was