Many Lactobacillus strains have been promoted as good probiotics for the prevention and treatment of diseases. We engineered recombinant Lactobacillus casei, producing biologically active canine granulocyte macrophage colony stimulating factor (cGM-CSF), and investigated its possibility as a good probiotic agent for dogs. Expression of the cGM-CSF protein in the recombinant Lactobacillus was confirmed by SDS-PAGE and Western blotting methods. For the in vivo study, 18 Beagle puppies of 7 weeks of age were divided into three groups; the control group was fed only on a regular diet and the two treatment groups were fed on a diet supplemented with either 1 x 10(9) colony forming units (CFU)/day of L. casei or L. casei expressing cGM-CSF protein for 7 weeks. Body weight was measured, and fecal and blood samples were collected from the dogs during the experiment for the measurement of hematology, fecal immunoglobulin (Ig)A and IgG, circulating IgA and IgG, and canine corona virus (CCV)-specific IgG. There were no differences in body weights among the groups, but monocyte counts in hematology and serum IgA were higher in the group receiving L. casei expressing cGMCSF than in the other two groups. After the administration of CCV vaccine, CCV-specific IgG in serum increased more in the group supplemented with L. casei expressing cGM-CSF than the other two groups. This study shows that a dietary L. casei expressing cGM-CSF enhances specific immune functions at both the mucosal and systemic levels in puppies.