Glycoprotein hormones LH, FSH, TSH and chorionic gonadotrophin are heterodimers composed of two non-covalently associated subunits, a common α- and a specific β-subunit. A recombinant baculo-virus containing a cDNA encoding the α-subunit of rat glycoprotein hormones was constructed. Viral-infected cells expressed, 48 h post infection, 7–10mg immunoreactive α-glycopolypeptide/6×108 cells, of which 65·6% was able to associate with native LHβ and formed a biologically active heterodimeric hormone that bound to testicular receptors. The treatment with specific glycanases showed that the recombinant α-subunit was produced as two differently glycosylated forms; an Mr 23 000 form which contained exclusively N-linked carbohydrate units and another of Mr 25 000 which appeared to contain additional O-linked carbohydrate. Data demonstrated that the α-subunit was expressed by insect cells in a manner similar to that by mammalian pituitary gonadotropes producing both the N- and O-glycosylated forms although only the N-glycosylated α-subunit is known to be capable of associating with the β-subunit.