Summary. We measured follicle-stimulating hormone (FSH) by both standard radio-immunoassay (RIA) and bioassay (Bio) methods in 56 men, using a modification of a previously described in vitro granulosa cell bioassay for the determination of the bioactivity of FSH. Thirty-four were young (mean age 25 years), and 22 elderly (mean age 72). No interfering factors such as disease or alcoholism could be identified. The elderly men had higher RIA-FSH levels compared with the younger ones. Bio-FSH demonstrated a similar trend and therefore the Bio/Immuno ratio was practically constant (3.3±0.6 vs. 2.7±0.4).We conclude that the age-associated decline in testicular function seen in normal ageing cannot be attributed to a chronic change in FSH activity. Not only is the RIA-FSH elevated among the elderly, it is of the same biological quality as in the younger men. The underlying cause for the testicular function decline is probably at the level of the testis itself.