The blood-monocyte chemotactic, phagocytic and microbicidal activities were studied in 20 patients with acute bacterial meningitis. The cell functions were measured on admission, during treatment and after clinical recovery. In general, monocyte functions were within normal range on admission. However, in 2/2 patients with Neisseria meningitidis meningitis, complicated by disseminated intravascular coagulation, defective chemotaxis was observed. Other patients had normal or enhanced monocyte chemotaxis. Two patients died: one had normal monocyte functions, one had initially defective chemotaxis and microbicidal activity. During treatment, cell functions normalized and after recovery all but one patient had a normal blood-monocyte function profile. In conclusion, acute bacterial meningitis is not due to, or followed by, abnormal nonspecific functions of blood monocytes. However, development of disseminated intravascular coagulation is associated with depressed chemotactic responsiveness.