Ficoll-Hypaque density gradient preparations of peripheral blood mononuclear cells from patients with systemic lupus erythematosus, rheumatoid arthritis, and acute rheumatic fever were highly "contaminated" with low buoyant density neutrophils. Plasma from these patients could induce an in vitro decrease of buoyancy in neutrophils with normal buoyant density. Similar change could be induced by complement-activated sera and aggregated gamma globulin. These data suggest that activated neutrophils are a common finding in the peripheral blood of these patients and may influence the interpretation of any studies with these cells. Functional studies of lymphocytes separated by Ficoll-Hypaque gradients should also take into account the higher degree of impurity of the cell preparations in patients with rheumatic diseases.