J. C. Servaites, in Plant Physiol. (1985) 78:839-843, observed that Rubisco from tobacco leaves collected before dawn had a much lower specific activity than the enzyme collected at noon. This difference persisted despite extensive dialysis, gel filtration, or heat treatment. However, precipitation of the predawn enzyme by 50% (NH4) 2SO4 restored the specific activity to the level of the noon-collected enzyme. Suggest an explanation.