Isoelectric focusing was applied to the Gc [group specific component] polymorphism. In agreement with Constans et al., 2 common subtypes of Gc1 [Gc1F and Gc1S were found] that could not be identified by conventional electrophoretic procedures. Gc1F has a slightly lower isoelectric point than Gc1S. In groups of US blacks the allele frequencies were for Gc1F; 0.732 and for Gc1S; 0.147. In whites these figures were 0.149 and 0.572. GcAb was also found in blacks with a frequency of 0.015. The concentrations in serum of Gc protein as measured by radial immunodiffusion did not differ according to phenotype.