A heterozygous state for C, deficiency was found in 14 members of a family with familial SLE. The HLA analysis in this family permitted us to re-establish the linkage assumed to exist between the gene (or genes) controlling the synthesis of the fourth component of the complement and the major histocompatibility complex. In addition, it was found that the complement deficiency is not necessarily associated with any serological or clinical abnormality. The heterozygous state for C4 deficiency can be added to the various serological abnormalities described in asymptomatic members of families with familial SLE. It is as yet unknown whether this deficiency may predispose to the subsequent development of SLE.