The histochemical nitroblue tetrazolium (NBT) test was performed in 88 patients with acute pulmonary thromboembolism or lobar pneumonia and in 88 healthy control subjects. In the control group, the mean NBT score was 5.4%. In the group of 55 patients with pulmonary thromboembolism the mean NBT score was 6.5% which is insignificantly different from the control mean. Only 3 of these patients had scores above normal. 35 embolism patients were followed by repeated examinations for 1 week after admission. The NBT scores remained within normal limits. 29 of the 33 patients with lobar pneumonia had elevated scores, and the mean NBT score was 33.5%. Endotoxin stimulated NBT test disclosed normal enhancement of NBT reduction by neutrophils from patients in either group as well as controls. The NBT test is of value in the differential diagnosis of pulmonary infection and thromboembolic disease. However, the results should be interpreted in the light of other pertinent laboratory and clinical findings.