Escherichia coli DNA polymerase IV encoded by the dinB gene is involved in untargeted mutagenesis. Its human homologue is DNA polymerase kappa (Polkappa) encoded by the DINB1 gene. Our recent studies have indicated that human Polkappa is capable of both error-free and error-prone translesion DNA synthesis in vitro. However, it is not known whether human Polkappa also plays a role in untargeted mutagenesis. To examine this possibility, we have measured the fidelity of human Polkappa during DNA synthesis from undamaged templates. Using kinetic measurements of nucleotide incorporations and a fidelity assay with gapped M13mp2 DNA, we show that human Polkappa synthesizes DNA with extraordinarily low fidelity. At the lacZalpha target gene, human Polkappa made on average one error for every 200 nucleotides synthesized, with a predominant T-->G transversion mutation at a rate of 1/147. The overall error rate of human Polkappa is 1.7-fold lower than human Poleta, but 33-fold higher than human Polbeta, a DNA polymerase with very low fidelity. Thus, human Polkappa is one of the most inaccurate DNA polymerases known. These results support a role for human Polkappa in untargeted mutagenesis surrounding a DNA lesion and in DNA regions without damage.