The R-factor R1drd-19 mediates resistance to beta-lactam antibiotics via a beta-lactamase. A strain of Escherichia coli K-12 carrying R1drd-19 was grown at different growth rates by using different carbon sources. The specific rate of production of the R1 beta-lactamase increased linearly with the growth rate and with the gene dosage. The content of R1 deoxyribonucleic acid was estimated by alkaline sucrose gradient centrifugation and by analysis of the specific rate of beta-lactamase synthesis in nutritional shift-up experiments and was found to decrease fivefold when the growth rate was increased from 0.4 to 1.8 doublings per h. The number of R1 molecules per cell decreased from six to two in the same growth range. The presence of the plasmid affected the mean cell size significantly; at a growth rate of 0.4 doublings per h the R-+ cells were on the average 50% bigger than the R-minus cells, whereas the effect was less than 10% at a growth rate of 1.8 doublings per h. Several reports in the leterature state that the initiation mass of chromosome replication is constant. In this paper it is shown that the initiation mass of R1 replication is proportional to the growth rate. Thus, the replication of the plasmid R1 and of the chromosome are independently regulated processes. It is argued that plasmid replication is under negative control.