In this short review of the characteristics of heterogeneous nuclear RNA of normal small lymphocytes and of leukemic blast cells the author points out that the average life time of the molecules of this RNA class in both types of cell is much longer than that so far reported for rapidly proliferating cells. In both types of cell the newly synthesized RNA includes rapidly hybridizing sequences. There is evidence, although circumstantial, that in leukemic cells these sequences are markedly different from that of normal lymphocytes. In both normal lymphocytes and leukemic blast cells the rapidly hybridizing RNA sequences are in part involved in secondary structure. In fact, a significant fraction of the hnRNA appears in RNase-resistant form with double-stranded properties. About one fifth of the hnRNA molecules of normal lymphocytes carries poly(A) segments, whereas this proportion is markedly higher in leukemic cells.