Further genetic heterogeneity of the transferrin system (Tf) was revealed by prolonged isoelectric focusing (PAGIF) of human sera on polyacrylamide gels (pH 4--6.5). One of the two common subtypes of TfC, designated previously as TfC1, is split into TfC1 and the new subtype, TfC3. The gene product of TfC3 has an isoelectric point between C1 and C2. In our sample (n = 252) seven phenotypes, TfC1, C2-1, C2, C3-1, C3-2, C3, and C1B2, were observed and the following frequencies calculated: TfC1 = 0.795, TfC2 = 0.155, TfC3 = 0.042, and TfB2 = 0.008. Family studies (n = 25) indicate an autosomal codominant mode of inheritance. The six TfC phenotypes are present after treatment of sera with neuraminidase and can be revealed in part by PAGIF and subsequent immunofixation.