Three males in a large family showed a selective deficiency of properdin (P). One of the P deficient individuals died from a fulminant infection with Neisseria meningitidis group C. The family history revealed three previous cases of similar infections with a fatal outcome. The deficiency did not appear to be associated with repeated bacterial infections. The pattern of inheritance suggested an X-linked mode of transmittance. However, heterozygous carriers were not clearly distinguished in the family. P deficient serum supported immune haemolysis in a normal fashion. Alternative pathway functions, such as the activation of C3 by inulin or zymosan, lysis of guinea-pig erythrocytes in agarose gel and opsonization of endotoxin coated oil particles, were grossly impaired in P deficient serum while efficient C3 activation was produced by addition of cobra venom factor.