The behavior of nucleoside triphosphates (NTP) in the RNA-polymerase system from E. coli was studied. The conversion of NTP to the corresponding nucleoside diphosphate (NDP) was observed. This phenomenon was accounted for a contaminating enzyme activity in the RNA-polymerase preparations. The possibility to remove such a contamination was demonstrated, the best technique being the DNA-cellulose affinity chromatography. The purified enzyme does not catalyze the intermediate formation of NDP's from NTP's during RNA synthesis with poly(U) and poly(dG)-poly(dC) as templates.