A modified Papanicolaou staining procedure using diluted Harris' hematoxylin with potassium alum is described. Nucleolar staining varies from blue to bright red. This technique has been applied to mammary tumor cell lines in vitro under several conditions of hormonal stimulation known to induce protein synthesis and cell differentiation. Blue nucleoli are observed in control resting cells, while bright red nucleoli are seen after hormonal stimulation.