A method for effective elimination of drug resistance (R) and sex (F) factors in Escherichia coli K-12 strains by treatment with sodium dodecyl sulfate (SDS) is presented. Growth of E. coli harboring R or F factors in Penassay Broth containing SDS led to the loss of all or part of these genetic elements. Appearance of drug-susceptible or F(-) cells among survivors was observed after the culture reached the stationary phase. Drug-susceptible cells which had lost all of their resistance markers by SDS treatment could be efficiently infected with R or F factors. Among isolated segregants which came from resistant cells, tetracycline-susceptible cells were the major segregant class. Drug-susceptible cells gave no revertants to drug resistance. By treatment of F(+) cells with SDS, unusual F(+) cells which retained mating ability but showed resistance to M12 phage were also isolated, together with mutants of another type which lost mating ability but retained sensitivity to M12 phage. Since SDS is more toxic to R(+) cells than R(-) cells, the isolation of drug-susceptible or F(-) cells under these conditions may be partly attributable to selective growth of drug-susceptible or F(-) cells in SDS-Penassay Broth.