The second case of C3b inhibitor deficiency is described in an 11-year-old girl who presented with recurrent attacks of meningitis, in between which she was well. Her serum showed all of the complement component changes noted in the first described case, although showing only a relatively slight defect in its ability to opsonize bacteria for phagocytosis and killing by polymorphonuclear leucocytes. This correlated with the patient's freedom from other infections.