With the use of Ag staining and conventional light microscopy, a difference is visualized in the frequency of nucleolar organizer regions between PHA-stimulated lymphocytes from normal females and females with various types of adenocarcinoma. This difference in NOR activity is more pronounced in G chromosome than D chromosomes. It is feasible that rDNA gene activity in PHA-stimulated lymphocytes is augmented in patients with adenocarcinoma due to the presence of plasma mediators arising from cellular malignancies. The NOR regions located on G chromosomes are considered to be more sensitive to these mediators than those on D chromosomes.