Four additional cattle blood group antigenic factors, provisionally termed F1, F6, F10 and F15, were shown to belong to the C system. Factor F1 appears to be a linear subtype of C" (initially designated F2, or P1B1). It is suggested that future international nomenclature should adopt C"1 and C"2 in place of F1 and C". No phenogroup was found to include C" together with C2 or C1, but a few phenogroups lack the three factors. Thus C1, C2 and C" do not form a closed system within the C system as concluded by Duniec et al. (1973). The effectiveness of the additional factors to uncover the genetic variability of the C system, and to translate phenotypes into genotypes is exemplified in the Charolais breed.