An abnormal subtelocentric, satellited chromosome which was found in a patient with chronic lymphocytic leukemia and in 50% of his family members was studied autoradiographically. It was demonstrated that this chromosome is an altered chromosome 13 with elongated short arms. The theories as to the origin of the extra chromatin were discussed in the light of the autoradiographic results. Some evidence is presented that other chromosomes of the D group may be involved in a similar anomaly.