The recombinant human prolactin was synthesised as an extracellular protein expressed in baculovirus system. The concentration of prolactin in TC-100 medium was approximately 40 mg/l when the conditions of recombinant virus infection were properly chosen. The human prolactin present in culture medium was stable at 4C for several months up to one year. The recombinant product was a survival factor for the insect cells. In the presence of prolactin in the medium, the cells did not show any signs of lysis or disruption, which is in agreement with the view of the antiapoptotic action of prolactin. The results of Western-blot analysis showed similar ratio of glycosylated/non-glycosylated forms of the recombinant product to the hormone forms present in human physiological (osmiotic) fluids. The recombinant protein was biologically active as determined in mammary gland explant system. The recombinant hormone present in the culture media was shown to induce mRNAs for two milk proteins ? beta-casein and WAP in mammary explants cultured in the presence of insulin and hydrocortisone. The effect of the hormone was dose-dependant and the largest accumulation of both mRNAs was observed at rec-hPRL concentration of 0.1 mug/ml (approx. 4.3 x 10 -9 M). In this respect, the activity of the recombinant human prolactin was equal or even higher than that of bovine pituitary prolactin or human growth hormone.