Delay-accelerating factor (DAF) protects host cells from complement-mediated damage by regulating the activation of C3 convertases on host cell surfaces. Using a panel of hamster-human somatic cell hybrids, the DAF gene was mapped to human chromosome 1. In situ hybridization studies using human metaphase cells further localized the gene to bands 1q31-41, with the largest cluster of grains at 1q32. This establishes the close linkage of the DAF gene to genes for four other proteins (C3b/C4b receptor or complement receptor 1, C3d receptor or complement receptor 2, factor H, and C4-binding protein) that share 60-amino-acid homologous repeats as well as complement-regulatory or -receptor activity, thereby enlarging the complement-regulatory gene family on the long arm of human chromosome 1.