The synthesis and the post-translational modification of the alpha subunit of human glycoprotein hormones have been studied in a mouse cell. A full-length cDNA coding for the human alpha subunit has been expressed in mouse C127 cells under the control of mouse metallothionein regulatory sequences, using a bovine papilloma virus vector. Stable clones secreting the alpha subunit into the medium have been obtained. Two intracellular forms of 22,000 Da and 21,000 Da have been detected. Pulse-chase experiments suggest that the 22,000-Da form is exported, while the 21,000-Da form appears to remain intracellular. The secreted form of the alpha subunit migrates as a broad peak between 22,000 and 30,000 Da, suggesting further modification of the intracellular form prior to secretion. Both the secreted and the intracellular forms incorporate glucosamine label, indicating that at least a portion of the modification observed here is in the form of glycosylation.