Phage phi 29 particles produced under restrictive conditions by mutants in gene 12 have normal amounts of all of the structural proteins except the appendage protein, p12*, which is missing. These particles are not infective and do not adsorb to Bacillus subtilis cells. By in vitro complementation of 12- particles with extracts containing protein p12* or with purified protein p12*, the defective particles could bind the appendage protein and become infective and able to adsorb to bacteria. Therefore, the neck appendages of phage phi 29, formed by protein p12*, are involved in the interaction of the phage with the cell wall receptors. Protein p12*, purified in its native state, competed with wild-type phage for adsorption to bacteria. Also, protein p12* could displace adsorbed phage from bacteria. Since the displaced phage was infective, protein p12* does not seem to be modified after phage adsorption.