The differential rate of synthesis of several Escherichia coli gene products was measured under conditions in which the average number of copies of the corresponding chromosomal gene had been changed by altering the replication velocity of the chromosome. The data show that in steady state exponential cultures the output of genes in a fully repressed, fully derepressed, or non-repressible state is proportional to the average number of copies of the gene per unit mass (gene: mass ratio) and does not depend on the number of copies of the gene relative to all other genes (gene: DNA ratio). In contrast, the output of a gene which was under regulation by endogenously generated effectors was independent of such changes in gene frequency. The relationship found between the number of copies of a gene per unit of cell mass and enzyme output provides a new method for determining the location of the chromosome origin and the direction of replication in bacteria.