Serum immunoglobulin concentrations and densities of IgA-producing immunocytes in intestinal mucosa were compared in a group of clinically healthy dogs of various breeds, a group of clinically healthy German shepherd dogs, and a group of German shepherds with bacterial overgrowth in the proximal small intestine. Serum concentrations of IgA, but not IgM or IgG, were significantly lower in the clinically healthy German shepherd dogs than in other purebreed and mixbreed dogs, indicating that production of IgA by gut-associated lymphoid tissue might be relatively low in this breed. However, densities of IgA-producing cells were not significantly different comparing these two groups, suggesting that any impairment of mucosal IgA production is more likely to be related to defective synthesis or secretion of IgA than to reduced numbers of IgA-producing immunocytes. Comparable findings in German shepherd dogs with small intestinal bacterial overgrowth provided further indirect evidence that local immunity might be defective in this breed, since these luminal bacteria would be expected to stimulate mucosal IgA production. However, it is not clear whether such a defect is directly responsible for the overgrowth, or whether there is an indirect relationship between defective local immunity and bacterial overgrowth in German shepherd dogs.