Chromosomes from a rat kangaroo (Potorous tridactylus) cell line (PtK2) and from a Chinese hamster (Cricetulus griseus) cell line (CHV79) were isolated by means of fluorescence activated flow cytometric sorting. DAPI (4'-6-diamino-2-phenylindole) was used as the DNA specific fluorescent dye. The karyotype of the PtK2 cells which exhibits 13 chromosomes was separated into 6, and the 22 chromosomes of the CHV79 cells were resolved into 11 fractions. DNA extracted from these chromosomal fractions was used for restriction enzyme digestion and blotting on nitrocellulose filters. The blots were challenged with gene probes corresponding to ribosomal RNA (18S and 28S) and small nuclear RNA (U1-snRNA) genes. The rRNA genes were exclusively assigned to chromosomes containing the nucleolus organizing region (in PtK2: X chromosome; in CHV79: chromosomes 4, 5, 6, and 11). - Solely the largest chromosomes in both cell lines hybridized with U1-snRNA indicating that these gene sequences are located on those chromosomes only. Further possible genetic and biochemical applications of this experimental system are discussed.