Chlorophyll belongs in a larger class of phytochemical plant pigments currently receiving more attention as a physiologically active dietary component. Although most research has focused on its biological activities such as its antioxidant, antimutagenic, anti-inflammatory or apoptotic effects in humans or rodents, there is limited knowledge at this time about the combinative possibilities of chlorophyll with probiotic bacteria. Our aim was to test the growth characteristics of canine-derived probiotic strain Lactobacillus fermentum CCM 7421 in the presence of different concentrations of chlorophyllin in vitro. Antimicrobial activity of chlorophyllin against canine indicator bacteria was also detected. In the in vivo study, chlorophyllin, L. fermentum CCM 7421 and the combination of both additives on faecal microbiota, faecal organic acid concentrations, haematological and immunological parameters in dogs were tested. Forty dogs were divided into 4 treatment groups; control (C); receiving chlorophyllin (60 mg/day/dog, CH group); L. fermentum CCM 7421 (10(8) CFU/day/dog, LF group); and both additives (CH + LF group), 10 dogs in each group. The experiment lasted for 28 days with a 14-day treatment period (sample collection at days 0, 7, 14 and 28). Results showed no growth inhibition of strain CCM 7421 by 0.05-0.25 % of chlorophyllin in broth after 24 h. Reduced growth of staphylococci, Listeria monocytogenes and Citrobacter freundii was observed at 1 % chlorophyllin (P < 0.05). In dogs, lower coliform bacteria numbers and higher concentration of propionic acid in faeces of the CH group during the treatment compared to baseline were detected (P < 0.01). Phagocytic activity of leukocytes was stimulated in all three treated groups of dogs (P < 0.05).