Using the technique of mRNA-cDNA hybridization, we have shown that there are between 11,500 and 12,500 different mRNAs in three different mouse tissues:kidneys, brains, and livers. Several experiments suggest that in each tissue the mRNAs are organized into three abundance classes rather than as a continumm with respect to concentration. Cross-hybridization experminets show that the most abundant class of mRNA in each tissue is characteristic, and that a high proportion of the total sequences are common between tissues. For a more complete analysis, cDNA was fractionated into three classes. Studies using isolated abundant cDNA show that some abundant sequences of liver and kidney are present in other tissues, but among the lower frequency classes. Thus tissue-specific differences in mRNA populations may be related to abundance as well as qualitative differences. Using isolated middle frequency cDNA of the kidney, it was shown that of the 550 or so sequences in this class, approximately 500 are shared with the liver. Similarly, between 9,500 and 10,500 of the low frequency kidney cDNAs are shared with the brain and liver, respectively, suggesting that the majority of mRNAs may be involved with "housekeeping" activities. In an attempt to see whether abundance of mRNA is related to repetition of the sequence in the genome, it was shown that abundant and middle frequency cDNA of the liver and kidney contain a component that anneals with DNA repeated approximately 100 fold. However, the low frequency cDNA of the kidney contains no repeated sequences.