IN this laboratory a method has been developed for measuring the luteotrophic activity of prolactin by the formation of deciduomata in a damaged uterine horn in the hypophysectomized adult mouse¹. The decidual response was quantal and the percentage of positive responses was transformed into probits. The method was designed as a 4 or 6 point parallel line assay and results were expressed in terms of the 2nd International Standard Prolactin (ovine prolactin, Armour Lab., Lot No. D 14083â 2B, potency 22 I.U./mg).