Peripheral blood mononuclear cells (monocytes) from patients with Whipple's disease in long-term remission were tested for their ability to handle intracellular microorganisms. Phagocytosis and lysis of Candida tropicalis by monocytes of patients (n = 12) and controls (n = 8) were quantified after 30 min of incubation. Phagocytosis was similar in both groups but intracellular killing of Candida tropicalis was significatively lower in patients (p < 0.001). We concluded that our study showed an in vitro defect in the intracellular killing function of monocytes in subjects in remission many years after diagnosis of Whipple's disease. The defective function did not seem to be related to relapse or to the susceptibility to other infections.