Follicle-stimulating hormone (FSH) preparations from various species stimulate the rate of formation of cyclic AMP by ovarian tissue, but there has been doubt whether this effect is due to an inherent property of FSH or to contamination of the FSH preparations with luteinizing hormone (LH) (Marsh, Mills & Lemaire, 1972). To answer this question we have made use of an antiserum directed against the β-subunit of ovine LH. Such antisera neutralize LH, but unlike antisera raised with intact LH do not cross-react with FSH and thyroid-stimulating hormone (TSH) which have a similar α-subunit (Pierce, Liao, Howard, Shome & Cornell, 1971). Ovine LH (NIH-LH-S18) purified on Sephadex G-100 was dissociated and separated into its α- and β-subunits as described by Reichert, Rasco, Ward, Niswender & Midgley (1969). The concentrated β-subunit fraction was emulsified with Freund's complete adjuvant and used for immunizing rabbits (twice 200 μg antigen/rabbit by multiple intradermal injections, 2