This paper studied the effect of intranasal inoculation of Lactobacillus fermentum, a microorganism belonging to the normal flora of the mouse pharynx, on the respiratory tract of mice. Optimal temporary colonization in different areas of the tract was obtained through administration of 4 times a dose of 5 x 10(7) CFU. L. fermentum remained in the trachea and bronchi up to the 7th day after inoculation. Re-inoculation of lactobacilli on the 10th day produced a transient colonization of the respiratory tract. Histological modifications produced in the trachea were mainly observed as an increased lymphocyte population at sub-mucosa level on the 4th day after inoculation. There was an increased number of activated macrophages in cytological slides of lung tissues on days 2 and 4. Re-inoculation also produced stimulation of the G2 macrophages on days 12, 14 and 17. From a histological point of view there were no other important changes in the organs studied. These results suggest stimulation of the immune system, especially that of the mucosal surfaces, after intranasal administration of L. fermentum in the experimental model employed. Stimulation was reflected in tracheal lymphocyte proliferation and increased lung macrophage population which have to be further studied in more detail.