Twenty-nine patients with Systemic Lupus Erythematosus (SLE) and 6 patients with Acute Poststreptococcal Glomerulonephritis (APGN) have been studied with the polymorphonuclear neutrophil (PMN) immunohistological technique to detect in vivo interaction between circulating immune complexes (IC) and PMN membrane receptors. Patients have been studied both at diagnosis and during follow-up and the results compared to those yielded by the C1qSP test. Our data provide evidence that the PMN immunohistological technique may prove a useful tool in monitoring IC disease. Moreover, elution studies may allow the detection and characterization of the antigens in the PMN-bound IC.