This study assessed the efficacy of a triple antibiotic paste in the disinfection of immature dog teeth with apical periodontitis. The canals were sampled before (S1) and after (S2) irrigation with 1.25% NaOCL and after dressing with a triple antibiotic paste (S3), consisting of metronidazole, ciprofloxacin, and minocycline. At S1, 100% of the samples cultured positive for bacteria with a mean CFU count of 1.7 x 10. At S2, 10% of the samples cultured bacteria-free with a mean CFU count of 1.4 x 10. At S3, 70% of the samples cultured bacteria-free with a mean CFU count of only 26. Reductions in mean CFU counts between S1 and S2 (p < 0.0001) as well as between S2 and S3 (p < 0.0001) were statistically significant. These results indicate the effectiveness of a triple antibiotic paste in the disinfection of immature teeth with apical periodontitis.