A search for familial visible chromosome markers has been initiated among couples who have lost several children by spontaneous abortion. Because the great majority of genetically unbalanced offspring of translocation heterozygotcs die during intrauterine development, repeated spontaneous abortions of unknown etiology, intermittent with successful normal pregnancies, can suggest a reciprocal translocation in the genome of one of the parents. Among the first ten couples studied, one abnormal karyotype was found in the male of a young couple. His wife had lost two children in the third month of pregnancy and has now an apparently successful pregnancy in advanced stage. The abnormal karyotype contained a chromosome of the 21 to 22 group which had an extra chromatin piece, of undetermined origin, translocated to its short arm. This chromosome was also found in the man’s fathe