抄録
Vitamin B_<12> requiring Escherichia coli mutants #215 and #113-3 occasionally revert to the prototrophic state during the serial transfers in an ordinary nutrient medium. The rates of reverse mutation of these cultures were determined according to the method reported by Lea and Coulson. Percentage of the culture containing non-reverse mutant was estimated utilizing the modified synthetic medium of Johansson, supplemented by limiting amounts of vitamin B_<12>, and the rates were estimated as 0.18-0.08×10^<-9> in #215 and 2.4±0.11×10^<-9> in #113-3.
