Rabbit anti-LH-S5 plasma was tested in the ovarian ascorbic acid-depletion (OAAD) assay against NIH-LH-B1, sheep anterior pituitary powder (SAP), chicken anterior pituitary powder (CAP) and chicken plasma. The anti-ovine LH destroyed the OAAD activity of LH-B1 and SAP but did not significantly affect CAP or chicken plasma. Agar immunodiffusion showed precipitin bands between theanti-ovine LH and LH-S5, LH-B1, porcine LH and SAP but not normal rabbit serum or CAP. When guinea pig anti-CAP plasma was allowed to diffuse on agar, bands formed with CAP and chicken plasma but not with LH-Bl or SAP. These results confirm and extend previous indications of a species difference between chicken and mammalian gonadotrophin. (Endocrinology80: 1182, 1967)