As a first step in quantifying gene expression during differentiation of the Vicia faba root cells we have analysed the mRNA population from meristematic cells. Using cDNA X mRNA hybridizations we have shown that this population can be divided into three abundance classes (abundant, intermediate and rate) representing 37%, 34% and 29% of the mRNAs and containing 26, 610 and 11700 sequences respectively. The total base-sequence complexity of the mRNA population, as determined by cDNA X mRNA hybridization, was found to be 1.6 x 10(7) nucleotides. This estimate was confirmed by the determination of the amount of single-copy genomic DNA hybridizing to the mRNA. Expressed as the number of structural gene transcripts, this complexity corresponds to about 13000 mRNA sequences.