The effect of a bacterial extract orally administered to 20 children with recurrent infections of the upper respiratory tract, was investigated in a double-blind study. The composition of the peripheral blood mononuclear cells (T and B-lymphocytes, monocytes) and some of their biochemical properties (5'-nucleotidase, beta-N-acetyl-glucosaminidase and non-specific esterase) were unaffected. In contrast, the allogeneic mixed lymphocyte reaction was significantly increased in patients treated with the bacterial extract. In the treated group the number of infectious episodes decreased significantly and the clinical response correlated positively with the mixed lymphocyte reaction. These findings suggest that the bacterial extract has the capacity of restoring depressed immune functions by acting through the gut-associated lymphoid tissue.