<p>rFSH is a Follicle Stimulating Hormone (FSH) preparation of recombinant DNA origin, which consists of two non-covalently linked, nonidentical glycoproteins designated as the &alpha;- and &beta; subunits. The &alpha;- and &beta;-subunits have 92 and 111 amino acids respectively, and their primary and tertiary structure are indistinguishable from those of human Follicle Stimulating Hormone. Recombinant FSH production occurs in genetically modified Chinese Hamster Ovary (CHO) cells cultured in bioreactors. Purification by immunochromatography using an antibody specifically binding FSH results in a highly purified preparation with a consistent FSH isoform profile and a high specific activity. The biological activity of rFSH is determined by measuring the increase in ovary weight in female rats. The in vivo biological activity of rFSH has been calibrated against the first International Standard for Recombinant Human Follicle Stimulating Hormone established in 1995 by the Expert Committee on Biological Standards of the World Health Organization. rFSH contains no Luteinizing Hormone (LH) activity.</p>