We report here a novel genetically determined polymorphism of a human urinary mucin which is demonstrable by the separation technique of SDS polyacrylamide gel electrophoresis, followed by detection with radio-iodinated lectins. The mucins are demonstrable using various lectins but the polymorphism is most easily recognized using peanut agglutinin and we therefore propose to designate this new genetic locus PUM (peanut-reactive urinary mucin). Four common alleles have been identified and an autosomal codominant mode of inheritance has been found in the families studied so far.