Canine pituitary extracts contained material which was capable of inhibiting the binding between antisera to different prolactins and labelled non-canine prolactins. Maximum inhibition of binding was observed in the system consisting of antisera to ovine prolactin and labelled rat prolactin. This system was used to investigate the immunological activity of canine pituitary extracts and partially purified fractions. A rabbit antiserum which was raised against a canine pituitary extract and had significant immunological activity in the above system, together with labelled rat prolactin, was used to develop an assay capable of detecting material in canine pituitary extracts. This material gave parallel dose-response curves to a canine prolactin-rich fraction, D864C and purified ovine and bovine prolactin.