Weak Lu9 antigen in one Lu:-6 member of a family.

. There is good evidence that the antigens Lu6 and Lu9 are controlled by alleles in the Lutheran complex locus, but a family is here described in which the propositus though Lu:-6, and therefore presumably homozygous for Lu9, reacts only weakly with anti-Lu9, while her Lu:-6 sister reacts strongly, as expected. The observation awaits understanding.