Fifteen members from a new family in which hereditary angioneurotic oedema exists have been studied with respect to serum concentrations of some components of the complement system. C'1q levels were found to be normal in all family members studied. C'1 haemolytic activity, previously reported to be decreased during attacks of oedema in some patients with this disease, was always within normal limits in our patients. Haemolytic C'4 activity was found to be subnormal in diseased individuals both during and between acute episodes, an abnormality which is useful as a diagnostic tool. Serum C'2 activity, previously used as a diagnostic indicator, was found to be normal in some affected individuals between attacks and only slightly decreased during attacks if assayed by a method which provides supplemental human C'4. Immune adherence (I-A) titres were consistently very low in patients with hereditary angioneurotic oedema during clinically evident episodes of oedema or abdominal pain. This finding along with I-A studies in C'2 deficient sera suggests that C'4 is the limiting factor in development of the immune adherence reaction in human serum. In addition, I-A provides a simple effective screening test for hereditary angioneurotic oedema during episodes of oedema or abdominal pain.