Two stable hybridomas producing antibodies (Mab 1 and Mab 2) to bovine prolactin and belonging to the IgG1 subclass have been prepared. The cross-reactivity of Mab 1 and Mab 2 with some structurally similar pituitary protein (human, pig, whale, rat prolactins, bovine and human somatotropins) using indirect immunoenzymatic assay, was studied. It has been shown that Mab 2 reacts specifically only with bovine prolactin whereas Mab 1 interacts with human prolactin and prolactins of different animals. The specificity of Mab 1 to human prolactin was confirmed by immunoradiodetection assay on nitrocellulose filters. The data obtained give evidence of the existence of at least two different sterically nonoverlapping epitopes: one of them is specific exclusively for bovine prolactin and the other one is common, i.e. extraspecific.