The depression of immune responses during measles infection has been recognized since the early 20th century. Acute bacterial infections are the most common cause of morbidity and mortality in measles. This study investigated the phagocytic and bactericidal or fungicidal capacities of polymorphonuclear cells (PMN) and monocytes during measles infections. When 14 children with measles were compared with 25 healthy controls, a significant depression in PMN and monocyte function was found. Of these 14 children, seven developed acute bacterial infections. These seven children were compared with 11 children who had acute bacterial infections without measles. It was found that PMN in patients with measles did not achieve an activated state, as expected with acute infections. Serum immunoglobulins were not altered in these patients. These results confirm previous reports of depressed phagocytic and bactericial capacities of PMN in measles, and further document that peripheral blood monocyte functions are also depressed in measles.