Chemiluminescence of alveolar macrophages was examined in 45 patients with and without immunodeficiency. Some of these patients were suffering from pneumonia. Chemiluminescence was reduced in patients with immunosuppression compared to those with normal immunofunction. Although this parameter of macrophage function was on the whole increased in pneumonia, the patients with a weak immunodefense presented with a markedly reduced increase in chemiluminescence. This explains why opportunistic infections occur in such numbers if there is an immunodeficiency.