The beta-glucosidase (E.C. 3.2.1.20) activity of tobacco cotyledons infected with P. tabacina was investigated using chemical and cytochemical methods in the early phases of the disease before the appearance of symptoms. It was shown that the beta-glucosidase in the tissue originated from the two components : plants and pathogen. The conidia and the hyphae of P. tabacina contain beta-glucosidase which is very active in the latter. The total beta-glucosidase activity of the infected tissues seems to be related to the amount of mycelium produced ; the tobacco cotyledons respond to P. tabacina infection by an increase in the beta-glucosidase activity in the cells penetrated by and adjacent to the pathogen. This response is triggered at very beginning of the pathological process and persist until its final stages. Experiments in the conditions of magnesium nutrition in which blue mould resistance genes are completely expressed, indicated that blue mould resistance mecha nisms are related to the activation (or the synthesis) of beta-glucosidase in the tissues of the host-parasite system.