SUMMARY: The lability of human pituitary follicle-stimulating hormone (FSH) to dilute hydrochloric acid was studied. Hydrolysis at pH 1·0 and 20 °C for 3 h abolished the biological activity of the hormone in vivo, but hydrolysis at pH 0·0 and 20 °C for 3 h gave rise to fragments of low molecular weight which retained immunological activity. In contrast, the weakest hydrolysis conditions which removed all the N-acetylneuraminic acid units were pH 2·0 at 80 °C for 50 min. Under these conditions, 75% of the fucose units but none of the other monosaccharide units of FSH were liberated. The results demonstrate the considerable lability of FSH to mild acid and provide an example of a glycoprotein, acid hydrolysis of which is unsuitable for removal of N-acetylneuraminic acid and fucose units selectively.