The effect of endodontic irrigants and dressings was tested on bacteria in bovine dentin specimens experimentally infected with Enterococcus faecalis, Streptococcus sanguis, Escherichia coli, or Pseudomonas aeruginosa. Standardized, cylindrical dentin test pieces were prepared and cleaned by ultrasonic treatment with EDTA and sodium hypochlorite. The specimens were infected with the test organism for periods up to 14 days, and the degree of infection into the tubules was monitored using Brown & Brenn stain, scanning electron microscopy, and culturing of dentin dust from sequential bur samples starting from the pulpal side. E. faecalis rapidly infected the whole length of the tubules; S. sanguis required up to 2 weeks for complete infection; E. coli only penetrated to some 600 microns, even after prolonged incubation periods. P. aeruginosa infected dentin quickly, but apparently in very low numbers. E. faecalis persisted for at least 10 d after withdrawal of nutrient support, whereas the other 3 organisms died within 4 to 48 h. Endodontic medicaments were applied to infected specimen for comparison of antibacterial potency. Camphorated p-monochlorophenol was generally more efficient than Calasept, and of the irrigants tested, iodine potassium iodide appeared more potent than sodium hypochlorite or chlorhexidine. The presence of a smear layer delayed, but did not eliminate, the effect of the medicaments.