The amounts of ribonucleic acid (RNA) polymerase (beta' subunits) and ribosomes (RNA), and the fraction of RNA polymerase actively engaged in transcription, were measured in Escherichia coli B/r as a function of growth rate. By an improved method of quantitating protein bands on electrophoresis gels, the systematic error and reproducibility of the RNA polymerase determination were estimated to be less than 15 and 6%, respectively. For a threefold increase in growth rate, the fractional synthesis of polymerase (relative to protein) increased 1.5-fold, whereas the fractional synthesis of ribosomal protein increased 2.2-fold. The decrease in the amount of RNA polymerase per ribosome with increasing growth rate is interpreted as an expression of the control of the transcriptional read-through from the genes for ribosomal protein, rplJ,L, to the adjacent genes for RNA polymerase subunits, rpoB,C. The number of active RNA polymerase molecules was determined from the synthesis rates of stable and messenger RNA and the known RNA chain growth rates. Comparison of active and total RNA polymerase indicates that the fraction of active enzyme increases from 20 to 30% in the range of growth rates between 0.6 and 2.0 doublings per hour. Possible causes for the inactive enzyme are discussed.