2-carboxy-D-arabinitol-1-phosphate (CA1P) bound to Rubisco either in leaf extracts or after purification can be displaced by SO₄^2- ions. Thus, treatment of leaf extracts with a buffer containing 200 mol m^-3 SO₄^2- displaces any bound CA1P and enables measurement of maximum carboxylation potential. In tobacco leaves, the activity following treatment with SO₄^2- ions ('maximal activity') is greater than the total Rubisco activity. The ratio of the two activities altered in a dynamic way with fluctuations in irradiance. Even in species which do not produce significant amounts of CA1P, the maximal activity greatly exceeded the total activity. Anion exchange separation of components in acid extracts confirmed the absence of CA1P in tobacco leaves harvested above an irradiance of 300 μmol quanta m^-2 s^-1, but the presence of another inhibitor of Rubisco. These results are consistent with the regulation of Rubisco activity by inhibitors other than CA1P which, like CA1P, can be displaced by SO₄^2-ions.