In a newborn female, an abnormal karyotype, 46,XX/47,XX,+mar/47,XX,+9, was found associated with several malformations. The marker chromosome was present in 70% of peripheral blood lymphocytes, and its size appeared to be less than half of the smallest chromosomes. Several differential staining methods provided no indication as to its origin. Chromosomes isolated from EBV-immortalized lymphocytes of the patient were fractionated on a FACS-440. The marker was resolved as a sharp peak in the region close to the chromosomal debris: its DNA content seemed to be close to 40% of chromosomes 21 and 22. About 580000 minichromosomes were sorted. In order to optimize cloning conditions, a pilot cloning experiment was performed on a pool of sorted chromosomes 9, 10, 11 and 12.