Effect of irradiance on both initial and total ribulose bisphosphate carboxylase (RuBP Case) activities was investigated in soybean (Glycine max cv. Will) leaves. Initial RuBP Case activity (IA), present in a leaf extract prepared at 0°C and assayed without prior activation, was used as an estimate of in vivo activity. Total activity (TA) was measured in leaf extracts after full activation in high levels of CO₂ and Mg²⁺. Observed large variations in RuBP Case IA and TA in preliminary study, which prompted the necessity for optimizing conditions for extracting and assaying the enzyme in soybean. In a series of experiments optimal assay pH, Ru3P levels, Mg⁺ levels, and CO₂ levels were determined. Stability of RuBP Case in leaves and extracts was also investigated; observed storage of leaf tissue or extract at 0°C maintained stability of RuBP Case activation state for hours. Modified assay procedure was used to measure Ru3P Case IA and TA in leaves of soybean plants grown in 4 different irradiances: 20%, 53%, 70% and 100% full sun (FS). TA expressed on a leaf area basis was significantly reduced with lower irradiance, the result of a reduction in leaf volume. Percent activation (%act, IA/TA) was similar for the higher irradiance treatments (approx. 94%), but lower (74%) in leaves of the 20% FS grown-plants. However, percent activation in the 20% and 100% FS grown-plants was similar when leaves were exposed briefly to similar light conditions. Concluded %act to be dependent on leaf irradiance level at the time of leaf removal and extraction and not influenced by irradiance level during growth. Diurnal response of RuBP Case IA and TA also investigated in soybean plants growing in field, greenhouse and growth chamber. In all 3 environments, both IA and TA increased with increasing irradiance. Diurnal changes in TA were not due to changes in level of Fraction 1 protein (RuBP Case). Concluded light may also have a role in increasing TA as well as IA of RuBP Case. / Master of Science