1) Significantly low serum complement level was observed in patients with chronic hepatitis, liver cirrhosis, myasthenia gravis, SLE, chronic glomerulonephritis, Hashimoto's disease, paroxysmal nocturnal hemoglobinuria and hyperthyroidism. 2) The measurement of CH50 in chronic hepatitis, liver cirrhosis and myasthenia gravis had the diagnostic values of these diseases but little to manage them. 3) Reduced CH50 level in SLE was characteristic in acute stage and correlated with immunoserological findings (LE cell preparation, ANF and DNA-Ab) but with urinary protein volume. 4) The early complement components (C1, C4, C2 and C3) in acute SLE were decreased, especially C4 and C2. 5) The recovery of low CH50 in SLE with steroid therapy was later than those of the other serological examinations. These observations suggest that measurement of CH50 in SLE is most usefull to manage this disease.