The effect of various doses of three mutagens, including N-nitroso-N-methylurea (NMU) and N-methyl-N-nitro-N-nitrosoguanidine, was studied on the effectiveness of culturing and survival rate of haploid and diploid mesophyll protoplasts of tobacco. The dependence of these processes on the physiological and genetical state of the cells was also studied. In other experiments, alterations in the quantity of DNA in the nuclei were studied during culture of haploid and diploid tobacco protoplasts, and the effect of NMU on this was also studied. The data gave a correct estimate of the sensitiveness of higher plant cells to chemical mutagens. Evidence was found of alterations in the organization of genetical material in protoplasts 4-5 cell cycles after the mutagenic treatments.