Five pituitary preparations of follicle stimulating hormone (FSH), namely NIDDK-oFSH-17, Bioscan oFSH, Ovagen, Folltropin-V and F.S.H.-P., were examined for biological activity in terms of their potency in an in vitro bioassay, receptor assay and heterologous radioimmunoassay and in terms of their metabolic clearance rates. In the three assays, Bioscan oFSH was the most potent (P < 0.05) (3- to 5-fold the potency of NIDDK-oFSH-17), with Ovagen being 25-50% the potency of the NIDDK standard (P < 0.05). Folltropin-V and F.S.H.-P. had the lowest potencies in all three assays. For each preparation, the ratio of activities between the assays was not consistent, suggesting that the preparations behaved differently in each assay. In 9 of 10 cases, potency estimates in the heterologous radioimmunoassay were greater than those in the in vitro bioassay or receptor assay. Polyacrylamide gel electrophoresis of the preparations showed banding consistent with the molecular weight of FSH, but also indicated that the preparations were contaminated with other proteins to varying extents. The half-lives of these preparations when injected into the bloodstream of mature female mice were 28.0, 8.6, 13.4, 11.6 and 17.4 min for NIDDK-oFSH-17, Bioscan oFSH, Ovagen, Folltropin-V and F.S.H.-P. respectively. The slopes of the decay rates were significantly different from each other (P < 0.05) except between Ovagen and Folltropin-V. The results of these studies show that a number of widely available FSH preparations have differing biopotencies. Moreover, the biopotency of a preparation in vitro is not related to its metabolic clearance rate, and not all FSH preparations behave identically in different assays. Measures of biopotency in vitro combined with those of metabolic clearance rate may provide useful information on the properties of FSH preparations used for research purposes and for superovulation of farmed livestock.