Human prolactin was expressed in insect culture cells by recombinant baculoviruses carrying prolactin gene cDNA placed under the transcriptional control of polyhedrin gene promoter of Autographa californica nuclear polyhedrosis virus. Preliminary:results of recombinant human prolactin expression as extracellular as well as intracellular product of baculovirus expression system were presented at the FEES Meeting in Nice, France, in 1999 (Abstracts, p. 288). In the present work prolactin was expressed as a hexahistidine-tagged fusion protein and recombinant protein was purified by metal affinity resin, Yields varied between approximately 20 and 35 mg/liter of medium. This recombinant prolactin was biologically active in Nb2 lymphoma cell proliferation assay and after simple purification could substitute for pituitary-derived prolactin.