Silver nitrate has been used to demonstrate the chromosomal location of ribo-somal cistrons in nine tissue-culture lines derived from human tumors of various pathological origins. Control individuals have a particular modal number (range 7–10) of D- and G-group chromosomes stained with silver. In the controls, 96.2 % > of the D- and G-group chromosomes that have a stalk show silver staining, while no relationship can be seen in acrocentric chromosomes without stalks. The tumor cells, whose modal chromosome numbers range from 42 to 68, possess variable numbers of acrocentrics (11–18). The number of chromosomes stained with silver, however, remained at control levels (range, 6–9). These data indicate that, in humans, silver staining may not identify all NORs that contain structural ribosomal