SUMMARYFactors concerned in demonstrating haemadsorption and haemagglutination and their occurrence among different mycoplasmas were investigated. Haemadsorption occurred best to colonies which had recently developed on agar at pH 6.5. Mycoplasmas isolated from various bird and animal sources, e.g. Mycoplasma gallisepticum, M. agalactiae, M. bovigenitalium and M . pulmonis, haemadsorbed with erythrocytes from a wide range of species. However, not all strains within a serotype haemadsorbed. Thus, the 'Negroni' strain of M . pulmonis did not. Haemadsorption could be inhibited by crowding of colonies on agar and by the addition of specific antiserum to the colonies. Generally, antiserum titres obtained by haemadsorption inhibition were low in comparison with those obtained by metabolic inhibition, and haemadsorption inhibition was not useful as a routine serological technique.The development of the haemagglutinin of Mycoplasma gallisepticum in liquid medium was studied in detail; a change in the pH value of the medium could be used as an index of its development and it was intimately associated with the organism. The centrifuged deposits of other mycoplasmas, from birds, cattle, goats, man, rodents and pigs, and which were grown in liquid medium also haemagglutinated, but generally to low titre. Haemagglutination occurred best in U-shaped cups at 37' and at pH 6.5-7-0 and could be inhibited by specific antiserum. There was lack of correlation between haemadsorption and haemagglutination; both these phenomena were exhibited by some mycoplasmas, others haemadsorbed only, and still others haemagglutinated only.