The investigation concerned 33 systemic lupus erythematosus (SLE) patients assigned to three groups representing mild SLE, more severe extra renal SLE, and SLE with significant renal involvement. In patients with extrarenal disease, the inflammatory plasma protein response was often pronounced during exacerbation, as evidenced by markedly increased concentrations of C-reactive protein (CRP), alpha 1-antichymotrypsin, alpha 1-antitrypsin, and orosomucoid. CRP responses were rare in patients with renal involvement, despite the increased concentrations of other acute-phase reactants in some of these patients. Superimposed bacterial infections were not clearly distinguished by raised CRP concentrations. The classical pathway of complement was activated in all patients during exacerbation, as indicated by increased concentrations of C1r-C1s-C1 inactivator complexes and C2a fragments. C1, C2, and probably also C3 activation varied according to the amounts of circulating C1q-binding immune complexes, as measured by solid-phase assay. Manifest hypocomplementemia was usually associated with glomerulonephritis. Participation of complement components in the inflammatory plasma protein response apparently counteracted the development of hypocomplementemia in many patients with extrarenal SLE. Circulating C3d was detected in all patients during exacerbation of renal disease and in most patients with severe extrarenal manifestations. Inverse relationships were found between immunochemical C2 concentrations and the percentage of cleaved C2 and between C3 and C3d. There was no appreciable consumption of factors B and D and properdin of the alternative pathway in the patients. High concentrations of factor D, a low molecular weight protein, were exclusively found in patients with renal involvement and could be ascribed to retention due to reduced glomerular filtration.