Measurements of the nine components of complement in the serums of 16 members of a kindred have established the diagnosis of hereditary deficiency of the second component of complement (C2). The autosomal recessive mode of inheritance resembles that of previously described families with C2 deficiency. Both C2 activity determinations with a stoichiometric haemolytic assay and C2 protein measurements with electroimmunodiffusion against antibody monospecific for C2 detect the heterozygous deficient state. Antigenic analysis, in vitro reconstitution experiments, and the constant ratio of C2 function to C2 protein indicate that the C2 synthesized by heterozygotes is indistinguishable from normal human C2. Studies of neonatal homozygous deficient serum and maternal heterozygous deficient serum show that transplacental passage of C2 does not occur. C2 deficiency in this family is not associated with clinical defects in host resistance.