The mRNAs of the mature pollen grain of Zea mays and of vegetative shoots were analyzed by comparing the kinetics of hydridization between homologous and heterologous reactions of cDNA to poly-(A)RNA in excess. The total complexity of pollen poly-(A)RNA i s 2.4×107 nucleotides, whereas in the seedling shoot it is 4.0×107 nucleotides. This corresponds to 24,000 different sequences in pollen and about 31,000 diverse mRNAs in shoots. The mRNAs in pollen can be divided into three abundance classes that constitute 35%, 49% and 15% of the total mRNAs having complexities of 2.1×105, 6.4×106 and 1.8×107 nucleotides, respectively. Estimates have been made of the number of copies of each sequence per pollen grain or shoot cell, and also of the pollen mRNA sequences shared with those of the shoot.