The interaction of prolactin with its receptor in the Nb2 cell line has been shown to induce the phosphorylation of cell-associated proteins and mitogenesis. It has been reported previously that one of these proteins, phosphorylated upon prolactin stimulation, was a tyrosine kinase. We have identified this kinase as JAK2, and demonstrate its association with the prolactin receptor. In addition, we show that the prolactin receptor itself becomes tyrosine phosphorylated upon ligand stimulation in Nb2 cells. These actions are time-dependent and occur rapidly after prolactin stimulation, with first the kinase being activated within 5 min and then the receptor being phosphorylated maximally at 20 min. Moreover, phosphorylation of both JAK2 and the receptor as well as Nb2 cell proliferation are dependent on the concentration of lactogenic hormone, resulting in a bell-shaped response curve similar to that observed in the two site model of hGH action. This indicates that early events in signal transduction as well as later events like mitogenesis and proliferation involve prolactin receptor dimerization. Together these data indicate that the prolactin receptor in Nb2 cells is associated to JAK2 and that upon ligand stimulation, and receptor dimerization, the kinase and the receptor are tyrosine-phosphorylated, which represents the first event in the process of prolactin receptor signal transduction in Nb2 cells.