Monocyte function in patients with cirrhosis of the liver was measured by phagocytosis and killing of Candida pseudotropicalis and C albicans. Both variables were significantly decreased in the patients compared with controls. Control monocytes exposed for two hours to patients’ serum showed a significant decrease in intracellular killing compared with control monocytes incubated in autologous serum. This suggests the presence of an inhibiting factor in the patients’ serum. This inhibitory factor passed through a dialysis membrane that permitted the passage of molecules of less than 12 000 daltons. Treating monocytes from patients with trypsin significantly increased phagocytosis, indicating that the possible inhibitory factor was attached to the monocyte surface. Metabolism of monocytes during phagocytosis as determined by chemo-luminescence was significantly lower in monocytes from patients compared with controls, indicating metabolic impairment. Monocytes are a component of the monocyte-macro-phage system, which includes Kupffer's cells. Impairment of the function of these cells, which have a pivotal role in clearing portal blood, might well be extremely important in the development of chronic liver disease.