Previously we showed that patients with IgA nephropathy present high serum levels of polymeric IgA and that in vitro polyclonal stimulation of their peripheral blood lymphocytes results in the synthesis of a large amount of true polymeric IgA. The aim of this study was to determine if the serum of patients with IgA nephropathy was capable of suppressing the directional migration of human normal polymorphonuclear cells (PMN), as do the polymeric fractions of IgA myeloma. Incubation of controls' PMN with fresh or heat-inactivated patients' plasma impaired the casein-induced directional migration significantly more than incubation with controls' plasma. This inhibitory effect was closely linked to polymeric IgA fractions and to a lesser extent with monomeric IgA immune complexes. The removal of IgA by immunoadsorption from patients' plasma completely abolished the migration suppression observed on controls' PMN. These results suggest that the high serum levels of polymeric IgA observed in patients with IgA nephropathy, by inhibiting directional migration and phagocytosis of PMN, and probably monocytes, could facilitate the persistent circulation and renal deposition of immune complexes.