A balanced reciprocal translocation involving the long arms of a No. 4 and a No. 11 chromosome, t(4;11) (q25;q13), was found in a mother who had one spontaneous first trimester abortion and 2 abnormal infants, as well as in her 2 phenotypically normal children. This translocation was not detectable by conventional karotyping since the two exchanged chromosome segments are almost identical in size; it was demonstrated by the new banding technic which allow us to recognize and to identify each chromosome and its segments accurately.