Four possible modes of action for the clinically observed effectiveness of sulfamethoxazole/trimethoprim in chronic granulomatous disease were evaluated: (1) inhibition of bacterial catalase, (2) improvement of granulocyte oxygen metabolism, (3) synergism of the antibiotic with nonoxygen-dependent granulocyte killing mechanisms, and (4) a purely antibiotic effect based on uptake and concentration of the antibiotic by and within granulocytes. While the first three mechanisms were excluded, the fourth mechanism is highly probable; sulfamethoxazole was found to reach granulocyte associated concentrations 1.7-fold and trimethoprim 4.1-fold of extracellular levels. Penicillin G, a known nonpenetrating antibiotic, reached 0.3-fold, and tetracycline, a known penetrating agent, 7.1-fold the extracellular level. These findings indicate that sulfamethoxazole/trimethoprim is an antibiotic combination uniquely suited for the long-term prophylaxis of infections in patients with defects of intracellular phagocyte killing.