SummaryStudies were made on the inhibitory effects of various substances on haemagglutination due to immune adherence of sheep erythrocyte‐antibody‐complement complexes (EAC′) to human erythrocytes. Two methods of measuring inhibition were tested. The more satisfactory of these was semi‐quantitative and involved determination of the number of EAC′ complexes, formed with a fixed amount of complement, required to produce immune adherence haemagglutination in the presence and in the absence of inhibitor. Ghosts prepared by lysis of normal human erythrocytes in distilled water or by freezing and thawing inhibited immune adherence haemagglutination. Ghosts from non‐reactive animal or human erythrocytes or from papain‐treated human erythrocytes did not inhibit immune adherence haemagglutination. Ageing of erythrocytesin vitrowas accompanied by loss of inhibitory power of ghosts prepared from them. Some mucoids extracted from human erythrocytes and most samples of mucopeptides extracted from human erythrocytes were inhibitory. Comparison of the inhibitory power of ghosts and mucopeptides indicated that considerable loss of activity occurred during extraction. It is suggested that the immune adherence receptor was partially degraded during extraction and that the mucopeptides acted as ‘incomplete analogues' of the receptor. It is considered more likely that the immune adherence receptor lies in the peptide part of such material than in the carbohydrate part. Neuraminic acid had no inhibitory activity. Active inhibitors were also extracted from bovine erythrocytes and human platelets, which do not react in immune adherence. The possibility is suggested that the immune adherence receptor may be present but masked or present in incomplete form on some non