Final seed protein concentration (SPC) is determined by the accumulation of protein, oil and residual fractions in the seed. As such, high SPC can result from more rapid and/or prolonged protein accumulation as well as a diminished accumulation of other seed components. The potential to increase SPC, therefore, depends on the relative responses of each seed storage component to assimilate availability. The objectives of this research were to (i) evaluate the accumulation patterns of protein, oil and residual mass in lines differing in final SPC, and (ii) assess the response of the major seed components to increased assimilate availability per seed during seed filling. Four closely related soybean lines (94% isogenic) selected divergently for high or low SPC were grown in the field along with their recurrent parent. One of the high SPC lines accumulated protein more rapidly than the recurrent parent, but had shorter seed fill duration. The other high SPC line had an intermediate rate of protein accumulation and a longer duration of seed fill, compared to the recurrent parent. Removing approximately 50% of the pods at R5.5 to increase assimilate per seed increased SPC in both high and low protein lines. The rate of protein accumulation was more responsive to the increase in assimilate availability than were oil and residual DM accumulation. Depodding the low protein lines increased final SPC to levels comparable to those of high protein lines in the control treatment. Evidently, seeds of high and low SPC lines had a greater capacity for protein accumulation than they expressed under normal growing conditions. These results support the hypothesis that SPC is under both zygotic and maternal control.