In sarcoma cells, the nucleoli are small, ill-defined, and are present in large numbers making it difficult to stain them by classical staining procedures. A modified Ag-NOR technique has been applied to detect the nucleolus and the nucleolar-organizing region in the chromosome of a mouse sarcoma-180 ascites cell-line. The cells were found to have 8-14 nucleolar chromosomes. The nucleoli are present in varying numbers corresponding to the number of nucleolar chromosomes. Silver grains appear as dots at one end of the chromosome. This end is presumed to be the centromeric end as, in normal mice, the NORs are in association with the centromere. However, the centromeres are ATrich, whereas the NORs should be GCrich. This NORs in mouse chromosomes should be placed below the centromere as already shown by the nucleic acid hybridization technique.