The complement titres of 290 specimens of serum taken from 70 patients with disseminated lupus erythematosus, acute glomerular nephritis, the nephrotic syndrome, anaphylactoid purpura, dermatomyositis and rheumatoid arthritis were determined by the spectrophotometric technic using the 50% hemolytic end-point. These titres were compared to the normal range, and changes in titre during the natural and therapeuticallyaltered course of these diseases were studied. Serum complement was found to be diminished in disseminated lupus erythematosus, acute glomerular nephritis and the nephrotic syndrome. In anaphylactoid purpura the titres were possibly slightly elevated and in dermatomyositis and rheumatoid arthritis, they were markedly elevated. During natural or induced remissions the abnormal titres returned toward the normal range. These findings are discussed in relation to experimental work and present hypotheses of the pathogenesis of these diseases.