Stably transfected cell lines expressing the α subunit, β subunit and α/β heterodimer of ovine (o)FSH have been established following the transfection of Chinese hamster ovary cells with α and β subunit cDNA expression vectors. In the absence of the α subunit, FSH β subunit polypeptides were inefficiently secreted and displayed a short intracellular half-life, while free α subunits were readily secreted in the absence of the β subunit. Cotransfection of oFSH α and β subunit cDNAs led to heterodimer assembly and secretion. While alteration of the nucleotide sequence flanking the β subunit AUG initiation codon did not appreciably enhance heterodimer biosynthesis and secretion, the replacement of the 5′ untranslated and signal peptide-coding regions of the β subunit cDNA with the corresponding sequences from an oGH cDNA clone was associated with a twofold increase in oFSH heterodimer secretion. The recombinant oFSH had a higher molecular weight than pituitary-derived oFSH, and was more acidic than the native hormone when analysed using isoelectric focusing, suggesting a greater degree of sialylation of the recombinant hormone. A comparison of the activities of the recombinant and native hormones in the porcine testis radioreceptor assay and in the in vitro Sertoli cell bioassay revealed that the recombinant oFSH displayed enhanced biological activity in the Sertoli cell assay when compared with the native hormone.