Greenhouse and field experiments were conducted to evaluate the effect of time, cultural conditions and genetic factors on protein extractability from tobacco leaves. Leaf protein was periodically extracted from green tobacco from time of transplanting through early senescence. Intensive sampling revealed substantial fluctuations in protein concentrations superimposed upon an age-related decline in protein levels with approaching plant maturity. Predictably, these oscillations affected the degree of protein extractability. Combined totals of the three major soluble protein fractions, viz. the green chlorophyll/protein complex, the white Fraction I protein (cryoprecipitate), and the white Fraction II protein (thermoprecipitate), deviated occasionally from protein values in unfractionated homogenates, but the fluctuation trends were similar. Investigations of variations in protein extractability among greenhouse-grown tobacco and selected genotypes grown under field conditions revealed that protein fluctuations during plant development are a common phenomenon. It is concluded that the occurrence of periodic fluctuations in total protein concentration and in the amount of extractable protein can severely distort estimates of leaf protein yields that are based upon insufficient numbers of samples and prolonged intervals between sampling.