Haemolytic activities of the classical and alternative complement pathways, and levels of C1, C4, C3, factor B and C1 inhibitor (C1-INH) were measured in 85 serum samples from 46 patients with chronic lymphocytic leukaemia (CLL). Significantly decreased mean C1 and C4 levels were found, and the haemolytic activities of these components were low or low normal in more than 50% of the sera tested. In 15 sera from 5 patients a complement profile characteristic of acquired C1-IHN deficiency was observed. These results indicate that the depression of the activity of the classical complement pathway is a frequently occurring feature in CLL.