Electrofocusing in agarose gel was used to investigate the distribution of subtypes of the group-specific component in a Danish population sample (n = 1,674). The frequencies observed were Gc1S = 0.5717, Gc1F = 0.1589, Gc2 = 0.2688. The rare allele Gc 1C3 was encountered with a frequency of 0.0006. Studies of 81 families with 198 children and 466 mother-child pairs confirmed the assumed three-allelic mode of inheritance. The application of Gc subtyping to paternity testing was investigated by examination of 208 cases involving 287 men. The reliability of exclusions of paternity performed by means of the extended Gc system is discussed with special attention to the occurrence of silent alleles.