Plasma postheparin lipolytic activity (PHLA) was measured on 50 patients with systemic lupus erythematosus (SLE). Plasma PHLA was significantly decreased in SLE patients. This decrease was most striking in the acute phase of the disease. There was a close relationship between decreased PHLA and immunologic factors indicative of the acute phase of SLE. These immunologic factors included shaggy antinuclear antibody pattern, low serum complement titer, high DNA antibody titer, mixed cryoglobulin and lumpy glomerular pattern by immunofluorescent staining.</p