The Plasma lipoprotein lipase (LPL, postheparin lipolytic activity, PHLA) was markedly decreased in active phase of systemic lupus erythematosus (SLE). And the most prominent decrement of the activity was observed in patients having "shaggy pattern" of the antinuclear factor. The LPL activity showed positive correlation with serum complement titer and nagative with anti-DNA antibody titer. Also the activity markedly decreased in patients with IgG-IgM mixed type of serum cryoglobulin. Furthermore, the LPL activity was decreased in the patients who demonstrated lumpy and granular pattern of the gromeruli by immunofluorescent technique. The plasma obtained from SLE patients inhibited considerably the plasma LPL activity of normal subjects.