Abstract This study used pharmacological intervention to provide support for a role of kinases and phosphatases in prolactin transactivation of a milk protein gene. It was based on transient co-transfection using a rabbit prolactin receptor expression plasmid and a β-lactoglobulin promoter/CAT reporter construct. In cotransfected CHO cells, herbimycin A and tyrphostin, two tyrosine kinase inhibitors, were able to decrease the CAT response by over 50%, along with tyrosine phosphorylation of cellular proteins, whereas genistein and lavendustine were without effect on lactoglobulin transactivation. Orthovanadate, an inactivator of tyrosine phosphatases, was able to substitute for prolactin in inducing the CAT response. Staurosporine, a non-specific kinase inhibitor, was able, when used at low concentrations (10 nM), to augment the prolactin response strikingly. Threonine/serine kinases do not appear to be involved early in β-lactoglobulin promoter transactivation, since four C-kinase inhibitors and okadaic acid, a threonine/serine phosphatase inhibitor, were without substantive effect. We conclude that specific tyrosine kinases are responsible for most of the signal transduction from the prolactin receptor to the β-lactoglobulin gene promoter.