Sera from a number of rhesus monkeys showed low or undetectable levels of LH according to radioimmunoassays which employ radioiodinated rhesus LH and antisera against rhesus LH or hCG. These same sera, when assayed by a system utilizing radioiodinated ovine LH and a unique anti-ovine LH serum which cross-reacts with LH from a variety of species, appeared to contain large and variable quantities of LH. The chromatographic behavior on Sephadex G-100 of the LH-like material in these sera was indistinguishable from that of authentic rhesus LH. Chromatographic fractions containing this LH-like material, as well as the sera from which they were derived, generated dose-response curves in the ovine:anti-ovine radioimmunoassay with steeper slopes than those produced by rhesus LH. These same chromatographic fractions had negligible activity in an alpha subunit radioimmunoassay which detects not only free rhesus alpha subunit but also the alpha component of undissociated rhesus glycoprotein hormones including LH. Treatment of these fractions with 4M guanidine-HCl produced a substance of smaller molecular size which, like rhLH beta, was active in the ovine:anti-ovine assay. A substance closely resembling the serum LH-like material but having a somewhat greater molecular size is also present in the rhesus adenohypophysis, but its relationship to the serum substance remains uncertain.