Abstract.Two individuals with the blood group Pkfound in the Japanese population are reported, as well as the results of family studies. A second Pksibling was found in the second family. Red cells from four members of the first family, who presumably were heterozygous for the gene responsible for Pk, showed no evidence of having a weaker P antigen. One of the two propositi gave birth to a baby, whose red cells gave a doubtful reaction in the direct antiglobulin test, but who showed neither clinical nor hematological signs of hemolytic disease of the newborn.