Humoral factors, phagocytosis, bactericidal capability, and ultrastructural morphology of blood polymorphonuclear leukocytes were investigated in 10 patients with Felty's syndrome. Five patients had suffered significant bacterial infections in the past two years while five other patients had not had bacterial infections. The patients with infections had higher concentrations of serum cryoglobulins, and their neutrophils contained immunoglobulin inclusions as seen by immunofluorescent and electron microscopy. Additionally, leukocytes from these patients showed decreased ability to kill Staphylococcus aureus and a minor defect in phagocytosis. There was a generalized subcellular disorganization of these inclusion containing neutrophils and a slower than normal release of lysosomes. Several of the above abnormalities may contribute to an increased susceptibility to bacterial infections in patients with Felty's syndrome.