Cytogenetic studies of 77 individuals, including 13 mongols, in six families are described. Chromosomal abnormalities which are segregating include a 13–15/21 type translocation (three families), a 21/22 translocation (one family), and a deletion of the short arm of chromosome no. 21 (one family). Clinical, biographical, and karyological data are presented. Segregation ratios are discussed. Serological data for eight blood group loci are given in all families in a search for linkage of these markers with the mongol chromosom