The group-specific component (Gc) subtypes were determined in 575 adult Finns by immunoblotting after isoelectric focusing in agarose gel. The gene frequencies were Gc1s= 0.661, Gc1F= 0.139 and Gc2=0.200. This material included one rare allele, a more acidically focusing Gc 2 (named Gc 2A18). The phenotypes of 200 mother-child pairs studied were in accordance with the three-allelic mode of inheritance. An apparent mother-child incompatibility observed during routine paternity testing is reported.