The 6-anilinouracils (AUs) constitute a new class of bactericidal antibiotics selective against gram-positive (Gr(+)) organisms. The AU family of compounds specifically inhibits a novel target, replicative DNA polymerase Pol IIIC. Like other antibiotics, AUs can be expected to engender the development of resistant bacteria. We have used a representative AU and clinically relevant strains of Staphylococcus aureus and Enterococcus to determine the frequency and mechanism(s) of resistance development. The frequency of resistance was determined by using N(3)-hydroxybutyl 6-(3'-ethyl-4'-methylanilino) uracil (HBEMAU) and commercially available antibiotics at eight times the MICs. For all five Gr(+) organisms tested, the frequency of resistance to HBEMAU ranged from 1 x 10(-8) to 3 x 10(-10). The frequencies of resistance to the antibiotics tested, including rifampin, gentamicin, and ciprofloxacin, were either greater than or equal to those for HBEMAU. In order to understand the mechanism of resistance, HBEMAU-resistant organisms were isolated. MIC assays showed that the organisms had increased resistance to AU inhibitors but not to other families of antibiotics. Inhibition studies with DNA polymerases from HBEMAU-sensitive and -resistant strains demonstrated that the resistance was associated with Pol IIIC. DNA sequence analysis of the entire polC genes from both wild-type and resistant organisms revealed that the resistant organisms had a sequence change that mapped to a single amino acid codon in all strains examined.