An attempt to induce some forward and back mutations in two Escherichia coli strains (his- and HfrH requiring thiamine) under the action of the carcinogenic nitrosamines--dimethylnitrosamine (DMN) and diethylnitrosamine (DEN)--is described. For this purpose the cells of E. coli were treated with 5% DMN or 1% DEN for 1 hour at 37 degrees C in 0.14 M NaCl. It was shown that the sensitivity of both strains to both nitrose compounds was not the same. DEN was 5-fold as toxic as DMN for the E. coli cells. DMN and DEN induced neither mutations of resistance to 10(-3) M valine, nor reversions in histidine-dependent strain. These mutations were obtained after the cells were treated with 0.1 M NaNO2. Lethal effects of DMN increased more than in 5 times and the toxicity of DEN did not change in hydroxylating mixture, in which nitrosamines derived to active compounds. Under these conditions both carcinogenes showed a mutagenic activity. DEN proved to be about twice as strong mutagenically as DMN. Thus, in our experiments we could see that DMN and DEN could induce both forward and back mutations in E. coli.