The steady state levels of mRNAs codying for the ribosomal proteins S6, S11, and S14 have been evaluated in quiescent and proliferating human fibroblasts and in resting and proliferating human peripheral blood mononuclear cells. It was found that the amounts of ribosomal protein mRNA are very similar and are not increased by serum or mitogen stimulation. The constitutive expression of these genes appears to be coordinately regulated and it is not modified after protein synthesis inhibition by cycloheximide. The ribosomal protein mRNA was also assayed in 15 different populations of human leukemic blast cells. In these populations the abundance of each ribosomal protein mRNA is remarkably different from the other. The results of our present experiments indicate that the expression of the three ribosomal protein genes undergoes independent noncoordinated changes in the large majority of the leukemic populations studied.