SUMMARY Immunological relationship between human pituitary luteinizing hormone (LH) and human chorionic gonadotrophin (HCG) was measured by microcomplement fixation, and a value of 1·17 was obtained for the immunological index of dissimilarity determined from the reactions of a highly purified human LH with rabbit antiserum to HCG. Treatment with neuraminidase of the LH preparation did not affect either the biological activity, as measured by ovarian ascorbic acid depletion, or the complement-fixing activity. The biological and immunological activities of LH were also not influenced by treatment with β-glucosidase or with neuraminidase plus β-glucosidase. Partial tryptic hydrolysis of LH caused decreases in both the biological and immunological activities, the greater loss being in the former. The biological and immunological activities of LH were nearly completely destroyed by treatment with urea, and removal of urea by dialysis did not restore the two activities. When the LH preparation was exposed to increased temperatures for various periods of time or to extremes of pH, the two activities were affected to variable degrees. These results indicate that the protein but not the carbohydrate moiety of the LH molecule is essential for both biological and immunological activities of the hormone and that the conformational integrity of certain parts of the protein moiety is also important for these activities.