Several mycoplasmas, from avian and mammalian sources, growing in the form of colonies on agar and sheets attached to plastic dishes, were tested for their ability to adsorb tissue culture cells in suspension. HeLa cells adsorbed to the majority of mycoplasmas tested; adsorption occurred to the sheets and not to the colonies of some mycoplasmas. Other tissue cells, in primary culture and of diploid origin, adsorbed also. The mechanism of adsorption of HeLa cells to 4 mycoplasmas was examined by treating the cells and mycoplasmas in various ways and then testing for adsorption. N-acetyl neuraminic acid residues on the tissue cells were responsible for adsorption to M. gallisepticum and M. pneumoniae. The receptors for M. hominis and M. salivarium were probably not of this kind since treatment of the cells with purified neuraminidase did not influence adsorption. However, the cell receptors for these mycoplasmas were associated with protein because they were inactivated by proteolytic enzymes and by formalin. The cell receptors for M. hominis were more heat stable than those for the other mycoplasmas. From the aspect of the mycoplasma membrane, in no instance did neuraminidase treatment affect adsorption. On the other hand, various experiments suggested that protein components of the mycoplasma membrane were involved. The significance of these findings is discussed.