Three A. bisporus strains (U1, C45 and B62) with different resistance towards bacterial blotch were infected with Pseudomonas tolaasii or with a partly purified toxin extract. In the two susceptible strains (U1 and B62), the total tyrosinase level decreased upon treatment with bacteria or the toxin-containing extract. The amount of active tyrosinase increased, suggesting an activation of tyrosinase upon infection. In the resistant strain (C45), the tyrosinase activity remained unchanged. Protease levels were found to be increased in regions of the sporocorp that were infected with the pathogenic bacterium, but not when treated with the toxin preparation, suggesting an activation of tyrosinase by a non-proteolytic mechanism. This observation was confirmed with in vitro experiments in which tyrosinase was found to be activated by a protease-free tolaasin-containing preparation.