The nature of the interaction of bacteriophage T4D and the outer cell wall of its host, Escherichia coli B, has been investigated. Bacteria with altered or modified cell walls have been obtained by two different growth procedures: (i) growth in high osmolarity medium or (ii) growth in broth in the presence of divalent heavy metal ions. When these altered host cells were washed and subsequently added to regular growth medium, they interacted with added phage particles, but successful infection did not occur. Most of the phage particles released from these treated cells were observed to have full heads and an altered tail structure. The altered phage tails had contracted sheaths and unusual pieces of the bacterial cell wall attached to the distal portion of the exposed phage tail tube. Phage released from bacteria grown in the high osmolarity medium had attached cell wall pieces of two major types, these pieces being either 40 or 21 nm in diameter. The smaller-type cell wall pieces (21 nm) were formed by three spheres each measuring 7 nm in diameter. Phage particles released from cells previously exposed to the divalent metal ions had only one 7-nm cell wall sphere attached to the distal end of the tail tube. It was found that these 7-nm spheres (i) are normal components of the cell wall and are morphologically similar to endotoxin, (ii) are held in place on the cell wall by a component of the cell wall called protein b, and (iii) are most likely the site of penetration of the phage tail tube through which the phage DNA enters the host cell.