Phage T5 requires 0.1 mM calcium to produce phage progeny in Escherichia coli cells. Decreasing calcium below 0.1 mM at time phage DNA was transferred depleted the bacteria of K+, caused membrane depolarization, perturbation of phage DNA transfer and resulted in a low internal ATP level. Our data suggest that calcium controls the conformation of the channel involved in the transfer of phage DNA through the host envelope and that below 0.1 mM calcium the channel remains open. This creates an energetic state of the host unfavorable to the synthesis of phage components and leads to abortion of the infectious process.