Objectives: The present in vivo study was designed to assess the efficacy of 3 root canal disinfection protocols in immature dog teeth with apical periodontitis (AP). Material and Methods: Forty immature premolars with pulp necrosis and AP of five Beagle dogs were used. Three experimental disinfection protocols were established. After irrigation with 40 ml 5.25% sodium hypochlorite using the Endovac system, in Group 1 canals were flushed with QMix solution; in Group 2, canals were flushed with QMix solution and 2% chlorhexidine gel dressing was placed for two weeks; and in Group 3, triantibiotic paste dressing was placed for two weeks. Canals were sampled after periapical lesions were radiographically visible (S1), after the first disinfection session (S2) and, in groups 2 and 3, after dressing (S3). Results: After the first session of the disinfection protocol (S2), there was significant (p < 0.05) bacterial reduction in the three experimental groups. Microorganisms were absent in 100% of S2 samples in groups 1 and 2, and in 75% of group 3 (p > 0.05). After dressing, 87.5% of the S3 samples showed increased bacterial count: in group 2, CFU counts (median = 891) were significantly higher than in group 3 (median = 18) (p = 0.03). Conclusions: In immature dog teeth with AP, root canal irrigation using QMix solution, with or without chlorhexi - dine gel dressing, or a triantibiotic paste dressing, provides the same level of disinfection than irrigation with 5.25% sodium hypochlorite alone in only one session.