The effect of oral bacteriotherapy with human Lactobacillus casei strain GG (10(10) colony-forming units twice daily for 10 days) was investigated in Crohn's disease and in juvenile chronic arthritis which are chronic inflammatory diseases associated with impaired mucosal barrier function. During oral bacteriotherapy, the gut immune response was indirectly assessed by solid-phase enzyme-linked immunoassay in 14 children with Crohn's disease, in 9 with juvenile chronic arthritis, and in 7 controls. The immunostimulatory effect of Lactobacillus GG was specific for Crohn's disease, irrespective of its activity: the mean (95% confidence interval) number of specific antibody secreting cells in the IgA class to beta-lactoglobulin increased significantly from 0.2 (0.04-1.3) to 1.4 (0.3-6.0)/10(6) cells and to casein from 0.3 (0.1-1.4) to 1.0 (0.2-4.8)/10(6) cells. The results indicate that orally administered Lactobacillus GG has the potential to increase the gut IgA immune response and thereby to promote the gut immunological barrier. Consequently, Lactobacillus GG could provide an adjunct nutritional therapy for Crohn's disease.