Seven ileally cannulated dogs were randomly assigned to a control or arabinogalactan (AG) treatments in a 7 x 7 Latin square design to evaluate effects of oral AG administration on nutritional and immunological characteristics. Arabinogalactan treatments included a high (1.65 g/d) and low (0.55 g/d) dose of AG100, AG1000 or AG3000 provided via gelatin capsules. Arabinogalactan forms differed in purification procedures. Each period consisted of a 6-d adaptation followed by a 4-d collection. Blood and fresh fecal samples were collected on d 10 of each period. Fecal score increased (P < 0.02) in dogs supplemented with the low dose of AG1000. Ileal and total tract dry matter (DM) and organic matter (OM) digestibilities were not affected by treatment. Dogs supplemented with the high dose of AG1000 tended (P = 0.15) to have a higher concentration of total aerobic fecal bacteria than control dogs. Dogs supplemented with the low dose of AG1000 and the high dose of AG3000 had higher concentrations of fecal lactobacilli (P = 0.04) and tended to have higher concentrations of fecal bifidobacteria (P < or =0.16) compared with control dogs. Dogs fed the low dose of AG3000 tended (P = 0.10) to have a lower concentration of fecal Clostridium perfringens compared with control dogs. Arabinogalactan treatments did not affect (P > 0.05) serum immunoglobulin G, M or A concentrations. Specific forms and doses of AG increased white blood cell, neutropil and eosinophil concentrations. Arabinogalactan is a unique dietary fiber that affects the digestive physiology and immunological characteristics of dogs.