Proteinase inhibitor activity was measured in tomato plants following infection with compatible and incompatible races of Phytophthora infestans.  Inhibitor activity in plant extracts was assayed for its inhibition of chymotrypsin and trypsin esterolytic activity.  Proteinase inhibitor activity increased rapidly and remained at a high level in tomato plants following infection with an incompatible race of P. infestans.  Inhibitor activity in tomato plants infected with compatible races either increased initially and then declined or it decreased from the time of infection.  In both cases, the inhibitor activity declined to below the initial level as the disease developed.  Infection of a single bottom leaf induced proteinase inhibitor activity in noninoculated upper leaves.  The increase in upper leaves was about two times greater after inoculation with the incompatible race than with compatible races.

Additional key words: chymotrypsin inhibitor, trypsin inhibitor, disease resistance.