We used two-dimensional gel electrophoresis to quantitate the changes in rates of synthesis that follow phage lambda infection for 21 Escherichia coli proteins, including groE and dnaK proteins. Although total protein synthesis and the rates of synthesis of most individual E. coli proteins decreased after infection, some proteins, including groE protein, dnaK protein, and stringent starvation protein, showed increases to rates substantially above their preinfection rates. Infection by lambda Q- affected host synthesis in the same way as infection by gamma+, whereas infection by lambda N- showed no detectable effect on host synthesis. Deletion of the early genes between att and N abolished the effect, and shorter deletions in this region gave intermediate effects. By this sort of deletion mapping, we show that a large part, though not all, of the effect of lambda infection on host protein synthesis can be ascribed to the early region that contains phage genes Ea10 and ral. We compared the changes in protein synthesis after infection with the changes that occur in uninfected cells upon heat shock or amino acid starvation. The spectrum of changes that occurred on infection was very different from that seen after heat shock but quite similar to that seen during amino acid starvation. Despite this similarity of the effects of lambda infection and starvation, we did not detect any increase in the level of guanosine tetraphosphate during infection. We show that the groE protein is the same protein as B56.5 of Lemaux et al. (Cell 13:427-434, 1978) and A protein of Subramanian et al. (Eur. J. Biochem. 67:591-601, 1976).