Transcobalamin II(TC II) is an essential transport protein for vitamin B12 in blood. TC II can be split up into isoproteins by polyacrylamide gel electrophoresis. Family studies are compatible with a genetic polymorphism of TC II and a five allele system. Screening of TC II isoprotein patterns in about 1000 individuals yielded two unusual TC II variants: The first case was a black female with severe megaloblastic anemia since infancy. Her TC II was elevated and bound B12, but displayed markedly diminished functional capacity to transfer radioactive cyanocobalamin in cellular systems. Comparison of this patient's TC II isoprotein pattern with known variants showed a distinct difference in electrophoretic mobility, indicating the presence of a sixth allele. The second patient, also presenting with pernicious anemia-like symptoms, was found to possess an unusual TC II variant with reduced TC II serum levels. Corresponding variants were also observed in the patient's asymptomatic children. Thus, abnormal TC II variants probably causing megaloblastic anemias both correlated with unusual isoprotein patterns.