Three cultivars of hard red spring (HRS) wheats with identical high molecular weight (HMW) glutenin subunit composition (5+10 type, Glu-D1d) but different dough properties and breadmaking quality were used in this study. The synthesis and accumulation characteristics of different protein fractions during grain development were examined. Samples were collected at three-day intervals from anthesis to maturity between day 10 to day 37. The nonreduced SDS-extractable glutenin aggregates of developing grains were characterized by a multistacking SDS-PAGE procedure to obtain information on the size distribution and polymerization of glutenin aggregates. The HMW to low molecular weight (LMW) glutenin subunit ratio was determined for its relationship to polymerization of the various glutenin aggregates of different molecular sizes. Glutenin proteins were quantified using an imaging densitometer. In addition, albumins and globulins, α- and β-gliadins, γ-gliadins, and ω-gliadins were separated by capillary zone electrophoresis. The results indicated that albumins-globulins, gliadins, and glutenins in developing grains were present at 10 days after anthesis or earlier. Albumin-globulins decreased in proportion, while gliadins increased in proportion during grain development. Polymerization of glutenin aggregates occurred 10 days after anthesis or earlier and increased significantly throughout the grain-filling period until maturity. Larger aggregates of glutenin increased in proportion, while smaller ones decreased in proportion during grain development. Ratio of polymers to monomers increased significantly from day 10 to day 22 of grain development and then remained constant until grain maturity. Glutenin polymers arrived at their maximum in proportion to total SDS-extractable proteins or monomers at day 22 after anthesis while the molecular size of these polymers continued to increase, as indicated by a rapid increase in proportion of HMW to LMW glutenin subunits. Significant differences were found in accumulation rates of glutenin polymers among the three cultivars. Cultivars Kulm and Grandin, with better breadmaking quality, appeared to have greater rates of accumulation and HMW subunit synthesis or formation of larger polymers than did Sharp, a cultivar with poorer quality. Significant differences were found among the three cultivars in the proportion of albumins-globulins and gliadins during grain development. However, no significant differences were found among the cultivars in the proportion of albumins-globulins, α-, β-, γ-, and ω-gliadins at grain maturity. Varietal differences in breadmaking quality were due mainly to the differences in glutenin polymers such as ratio of polymeric to monomeric proteins, molecular size distribution, and ratio of HMW to LMW glutenin subunits among wheat cultivars of 2*, 7+9, and 5+10 subunit types. The better breadmaking cultivars might be characterized with higher proportions of glutenins and greater proportion of HMW subunits in total SDS-extractable proteins than the poorer quality cultivar. However, more genotypes need to be examined.