Summary Serum hæmolytic complement has been measured in 166 patients. Low levels of complement occurred in systemic lupus erythematosus, in the nephrotic syndrome, and in parenchymal liver damage. High levels of complement occurred in obstructive jaundice and in conditions characterized by tissue destruction. The low serum complement level in systemic lupus erythematosus was attributed to complement‐fixing antibodies against human tissues found in that condition.