The kinetics of hybridization of polyadenylated RNA from mouse L-cells with complementary DNA (cDNA) synthesized with reverse transcriptase revealed three classes of differing abundance. The simplest interpretation requires three frequency classes representing polyadenylated RNA; 5, 45, and 50 percent of the total polyadenylated RNA and about 3, 300, and 7600 different RNA sequences of 6 times 10-5 daltons, respectively. The complementary DNA synthesized with L-cell polyadenylated RNA as template hybridized efficiently with RNA from different mouse tissues, indicating that most species of the L-cell RNA in the highand middle frequency class are present in all mouse tissues. Kinetics of hybridization of complementary DNA synthesized with cytoplasmic polyadenylated brain RNA as template suggested a higher complexity for brain RNA. Thirty-five percent of this brain cDNA failed to hybridize with L-cell RNA. This complementary DNA fraction, isolated by hydroxylapatite chromatography, represented approximately 11,000 RNA sequences specific for the brain. On the other hand, hybridization of complementary DNA synthesized on polyadenylated mouse liver RNA with L-cell RNA failed to demonstrate differences between these two groups of polyadenylated RNA.