A new method of C 3 type determination on high voltage agarose gel electrophoresis is described. The influence of some electrophoretic variables on the results has been investigated and a standard technique developed. The method is quick, simple and gives good separation of C 3 proteins. The three common C 3 phenotypes and the four variant types so far studied are easily demonstrated. Preliminary allele frequencies in a Norwegian population sample is presented. The values found, C 3^s = 0.80 and C 3^F = 0.19, are in good accordance with the allele frequencies in two population groups of North American Caucasians.