Inherited deficiencies of complement components are associated with an increased risk of infection by encapsulated, high grade bacterial pathogens such as Streptococcus pneumoniae, Haemophilus influenzae type b and Neisseria meningitidis. Hence, the levels of antibodies to bacterial capsular polysaccharide antigens were measured using ELISA in 65 patients with inherited deficiencies covering the classical, alternative and terminal components of the complement cascade. Three of the four C3-deficient individuals studied were found to be almost totally deficient in specific anti-pneumococcal capsular polysaccharide (PCP) antibodies. These individuals had a history of recurrent pneumococcal sepsis. While single individuals with C1r, C2 and C1Inh deficiency were found to have low anti-PCP antibody levels, no other group of complement deficiency had significantly reduced anti-PCP antibody levels compared with 100 controls. Antibody levels to the other two polysaccharides were not significantly lower in the patient groups. These findings suggest that C3 may be able to provide a stimulatory signal to promote the production of anti-PCP antibodies.