In both C'1q protein and agglutination assays, markedly reduced mean titres (27 and <10% normal, respectively) were observed in Swiss type lymphopenic agammaglobulinaemia. Mean C'1q titres were somewhat decreased in other forms of agammaglobulinaemia, but not to the same degree (78 and 46% normal, respectively). C'1q titres were normal or elevated in other diseases associated with repeated infections and malignancy. We conclude that the C'1q globulins, like the immunoglobulins, characteristically are deficient in agammaglobulinaemia syndromes. Since the greatest C'1q deficiencies were seen in lymphopenic agammaglobulinaemia of the Swiss type, titration of C'1q seems to be a convenient aid in diagnosing this disease early in life. Thus, the inborn error of agammaglobulinaemia seems to be reflected in the complement system.