This paper describes the construction and characterisation of a cDNA library from wheat endosperm tissue during the early stages of grain filling. Developing wheat endosperm tissue was characterised with respect to standard measures including dry weight, cytological appearance and timing of expression of major sources of mRNA such as the seed storage protein genes. In addition, the full complement of proteins present at mid-endosperm development was examined using 2D-electrophoretic techniques. Based on this characterisation, endosperm from the developing grain 8-12 days post-anthesis was chosen for isolating mRNA and preparing cDNA. At this stage in development the mRNA population is not yet dominated by the accumulation of mRNA from seed storage protein genes. A cDNA library, not normalised, containing a high percentage of full length cDNA clones was constructed and 4,319 clones sequenced ("single-pass"). Partitioning of the cDNA sequences into gene families and singletons provided the basis for quantifying the accumulation of sequence classes relative to the total number of sequences determined. The accumulation of gene families/singletons was not linear. However, mathematical modeling of the data suggested that the maximum number of different genes expressed is within the range of 4,500-8,000 (detailed in the Appendix). If an average is taken of these extremes, approximately 27% of the gene products were visible as proteins in the 2D-electrophoretic analysis. Analysis of a functional class of genes relevant to wheat grain end-use, namely the glutenin/gliadin seed storage protein class of genes, revealed a new category of gene characterised by a distinctive N-terminal domain and a reduced central repetitive domain.