SUMMARY Endotoxin-stimulated nitroblue tetrazolium (NBT) reduction was evaluated in 853 individuals: 270 healthy controls, 334 with various non-neoplastic conditions, 220 with solid non-lymphomatous tumours, and 29 with lymphoma. Each of the above groups was divided into three age subgroups: < 60, 60-69, and > 70 years.In the controls and in patients with nonmalignant diseases, significantly lower values were recorded for elderly subjects (. 60 years) compared with younger subjects of the same group, whereas in cancer patients the results were independent of age. Under the age of 60, stimulated values for both patient groups were significantly lower than the control values, and in patients with solid non-lymphomatous tumours significantly lower values were attained than in the other patients. NBT reduction in lymphoma patients was comparable to that of the controls. In the elderly (age > 70) no significant differences were noted between patients and controls.It is suggested that stimulated NBT reduction declines with advancing age. While this test clearly demonstrates some leucocyte dysfunction in solid cancer, its value in investigating neutrophil behaviour in elderly subjects is questioned.According to recent observations made in our laboratory, peripheral polymorphonuclear leucocytes (PMNs) from patients with solid cancer have a diminished capacity to reduce nitroblue tetrazolium (NBT) dye to formazan when maximally stimulated with endotoxin (Haim et al., 1977a(Haim et al., , 1977b