Studies on the growth kinetics of R+ and R− cultures of Escherichia coli in the presence of nalidixic acid (NA), acriflavine (AF) and kanamycin (Kan) showed that each drug caused a decline in viability of both R+ and R− cells for several hours. During further incubation the viability rose rapidly for the R+ cultures, but either rose less rapidly (AF and Kan) or continued to decline (NA) for R− cultures. Distribution curves of the resistances of individual clones of R+ and R− bacteria to atabrine, NA, AF and Kan suggested that the presence of an R factor in the host bacterium increased its mutation rate to resistance to these drugs: this would account for the more rapid growth rate of R+ cells during the latter stages of incubation in their presence. The mutations causing increased resistance to NA and to Kan were located in the bacterial chromosome and not in the R factor.