Polarographic reduction was studied for a series of 7 urea derivatives and the results were used to assess their potential carcinogenity. The polarographic reduction was examined in absolutely anhydrous dimethylformamide by DC polarography. In the conditions applied, the majority of the compounds was reduced within a single two-electron step, only biuret and its formyl derivative were reduced in two one-electron steps. The potential carcinogenity of the substances was assessed based on the tg α value of the slope of dependence of the polarographic wave height on the concentration of α-lipoic acid added as a test substance. For hydroxyurea, which is the only substance in this series for which a carcinogenic activity has been demonstrated, the tg α parameter attained a value of 0.290. Still higher values were obtained for the formyl derivatives - formylbiuret (0.362) and 2-carbamoyl-1-formylguanidine (0.510). So high tg α values warn of a significant potential carcinogenity. The other substances studied exhibited considerably lower tg α values, indicating that their potential carcinogenity will be low.