Films of regenerated Bombyx mori silk are strongly affected by absorbed moisture, a phenomenon studied here by differential scanning calorimetry (DSC). Exposure of previously dried films to environments of controlled relative humidity produces test samples of well-defined equilibrium moisture content. Ultimate moisture uptake is as high as 20-23% (by weight) at 75% relative humidity. The glass transition temperature, T-g, drops by 40 degrees C at moisture uptakes as low as 2%, and T-g depressions as large as 140 degrees C are observed at higher relative humidity. The moisture-induced decrease of T-g is completely reversible, as a film remoistened and then red-led possesses an unchanged T-g. Trends in T-g with water uptake correspond reasonably well to predictions of a classical thermodynamic theory, indicating that the plasticization effect of moisture on the combined silk-water system can be satisfactorily explained from macroscopic properties of the constituents without any reference to specific interactions.