Net photosynthesis on a leaf area and leaf weight basis increased significantly with ploidy in a 4X, 6X, 8X and 10X allopolyploid series of tail fescue (Festuca arundinacea Schreb.). Total protein did not increase significantly with ploidy. Rocket immunoelectrophoresis was used to quantitate ribulose-1, 5-bisphosphate carboxylase (RuBPCase) protein. RuBPCase content, expressed on both a concentration basis and as a percentage of total protein increased significantly with ploidy in both field and greenhouse experiments. The range of RuBPCase content was 16 to 73% of total protein and 2.8 and 6.5 mg/ml of extract. Specific activity of RuBPCase did not increase significantly with ploidy. Chlorophyll concentration increased as a quadratic function of ploidy, with the mean for 8X genotypes representing maximal chlorophyll content. Evidence is presented that increasing concentrations of RuBPCase are associated with higher net photosynthesis rates in tall fescue. This suggests that RuBPCase may represent a marker for increased net photosynthesis. RuBPCase was extracted in a partially active state or inhibited state and must be fully activated by Mg(2+) and HCO(3) (-) to measure maximal activities. Polyploidization appeared to increase selectively the allocation of total protein for synthesis of RuBPCase; however, there was also a range for carboxylase content among the genotypes within a given ploidy level.