The effects of lincomycin on the immune system were studied on patients suffering from chronic bronchitis by means of phagocytosis, chemotaxis and natural-killer tests. The tests were performed before and 2, 4, 8, 12 and 24 h after administration of 600 mg lincomycin i.m. in a single dose. The results indicate that lincomycin stimulates phagocytosis, expressed as enhanced superoxide production (O2-), chemotaxis and the activity of natural killer cells, measured on the basis of their ability to perform a lysis on the K 562 tumoral target labelled with 51Cr. The stimulating effects on chemotaxis appear already 2 h after the administration of the drug, while the same effect on phagocytosis and natural-killer activity occurs after 4 h. The maximal stimulating activity on all three parameters can be shown at 8 h and disappears at 24 h.