By use of poly(dA-dT) as template and Escherichia coli RNA polymerase, several metal ions were tested for their effect on the efficiency of transcription and on the misincorporation of CMP into the poly(rA-rU) product. In the presence of 10 mM MgCl2, Mn2+ has a stimulatory effect on the transcription, Co2+ has very little effect on the reaction, Cu2+ and Zn2+ are strongly inhibitory, and Cd2+ and Ni2+ are less inhibitory. The background misincorporation of CMP in the presence of MgCl2 is about 1 nucleotide per 2000 correct nucleotides incorporated and is independent of Mg2+ concentration. Zn2+, Ca2+, Sr2+, Li+, Na+, and K+--all nonmutagenic and noncarcinogenic--do not increase misincorporation. Mn2+ causes a concentration-dependent threefold increase in the misincorporation that can be slightly reversed at higher MgCl2 concentrations. Cd2+ causes a dramatic increase in the misincorporation with increasing CdCl2 concentration that can be substantially overcome by higher concentrations of Mg2+. Cu2+ also increases the misincorporation, Ni2+ slightly increases it, and Co2+ does not increase it at all. Several control experiments indicate that the misincorporation of CMP is dependent on the template-directed synthesis of poly(rA-rU). Nearest-neighbor analysis indicates that CMP is incorporated in place of UMP into the poly(rA-rU) product. The increase in misincorporation appears to be related both to the "hard-soft" character of the metal ions and to their carcinogenic potential.