Abstract Several basic parameters related to the synthesis of protein and RNA in cells of Escherichia coli B infected with bacteriophage T4 have been measured. (a) The over-all rate of protein synthesis, estimated by incorporation of four different amino acids, decreases to 42% shortly after infection. (b) A general method was developed for measuring the average peptide chain growth-rate under steady-state conditions. It involves separation on sodium dodecyl sulfate—acrylamide gels of total protein labeled for different short periods of time, followed by an analysis of the labeling kinetics of proteins in different molecular weight classes. By this method the peptide chain growthrate in T4-infected cells was found to be 5.6 amino acids per second at 30 °C compared to 11.4 amino acids per second measured in exponentially growing cells under the same growth conditions. (c) The proportion of ribosomes in polysomes before and at different times after T4 infection was determined by sucrose gradient analysis. No significant differences were observed. (d) The stimulating activity of RNA extracts in an in vitro protein-synthesizing system was measured and used as an estimate of messenger RNA. By this criterion the level of mRNA is at least as high and probably higher after infection. (e) Residual protein synthesis was followed after addition of rifampicin, an antibiotic that blocks initiation of RNA synthesis, and under these conditions the protein-synthesizing capacity decreased with a half-life of 2.2 minutes in uninfected, and 4.0 minutes in T4-infected cells (at 30 °C). The decrease in over-all rate of protein synthesis observed after infection is thus accounted for by the decrease in peptide chain growth-rate. The twofold decrease in peptide chain growth-rate compared with the twofold decrease in nucleotide chain growth-rate observed after T4 infection ( Bremer & Yuan, 1968 a,b ) shows that there is a close correspondence in the rates of the two processes, as in exponentially growing cells; however, the rates of chain elongation and of mRNA decay decrease about twofold after infection.