Irreversible adsorption of a virulent phage, phage A25, to heat-killed streptococci, groups A, G, and A variant, has been achieved. Adsorption reflected the observed host range for phage A25 in that heat-killed group B cells were not able to inactivate the phage. Broken cells, cell walls, and peptidoglycan prepared from a group A strain K56 failed to adsorb the phage irreversibly, but retained the potential to carry out reversible adsorption. Experimental data including electron microscopy have demonstrated the specificity of reversible adsorption and have identified the peptidoglycan as a necessary cellular component of the receptor. The sensitivity of whole cells and purified peptidoglycan to muralytic enzymes suggests that the cell wall and peptidoglycan must be intact for optimal adsorption. In general the results are explained by postulating that adsorption of A25 phage particles to group A cells occurs by a two-step process; the first step involves recognition and reversible binding of the phage tail to the cell wall peptidoglycan, the second step is an irreversible reaction catalyzed by a yet unidentified cellular component which is destroyed when cells are ruptured.