Abstract The first step in Haemophilus influenzae infections is the adherence of the bacteria to oropharyngeal epithelial cells. Those strains which adhere are also able to agglutinate human red cells. All adult erythrocyte samples were agglutinated by H. influenzae, but cord erythrocytes were negative. Subsequent testing of selected adult erythrocytes showed that the Anton antigen was essential for haemagglutination by H. influenzae from cerebrospinal fluid and sputum. This was shown by direct agglutination of Anton positive cells and inhibition studies using three anti-Anton sera.