Zymosan stimulated oxygen metabolism was investigated in polymorphonuclear leucocytes (PMN) from 6 diabetic patients. Oxygen uptake and superoxide production were continuously measured in the presence of autologous or control serum and non-opsonized zymosan, or in the absence of serum and preopsonized zymosan. The only significant impairment in the diabetic cells studied was a lower oxygen uptake in the presence of autologous serum. This defect was normalized by addition of control serum or by omitting the serum and stimulating with opsonized zymosan. In the absence of serum, the oxygen consumption was markedly diminished in only one subject, whereas two subjects showed a decrease of superoxide production in the presence of control serum. An inverse correlation between fasting glucose concentration and oxygen uptake could be demonstrated. However, exposure of normal PMN to hyperglycemic glucose concentration in vitro did not significantly alter their oxygen metabolism, suggesting that glucose alone could not be the only factor responsible for the impaired oxygen consumption in diabetic cells.