The commercial preparation of Bacillus subtilis spores may be considered within the classification of biological response modifiers (BRM's) and included among exogenous natural substances. Recently we decided to study the effect of a long-term B. subtilis spores oral treatment in children suffering from recurrent infectious diseases of the respiratory tract. Fifty-three children 5-9 years old have been studied. The clinical valuative parameter was the number of days of absence from school during a 4-month period. In another group of 12 diseased children, mean age 5.5 yrs we recently initiated a laboratory immunological evaluation of peripheral lymphomonocytes in relation to an oral treatment with B. subtilis spores for at least 2 months. Our results show that B. subtilis spore therapy significantly reduced the frequency of respiratory tract infections in the group of treated children. In addition, preliminary immunological laboratory evaluation demonstrated a complete return to the normal lymphomonocyte status after at least 2 months of treatment with B. subtilis spores.