Several varieties or species of tobacco were artificially or naturally contaminated with Peronospora tabacina A., at various stages of growth : 10 to 20-day-old seedlings (cotyledon test), greenhouse-grown plants (10 weeks old) and field-grown plants. Their soluble proteins were extracted and analysed by electrophoresis on polyacrylamide gel (4/30). The b proteins were evidenced in healthy plants : in seedlings at the cotyledon stage exclusively fed with magnesium nitrate (Mg2+) on the one hand, and in fully grown field plants once flowering and senescence processes had been set off. At the three stages of growth studied, P. tabacina contamination lead to the formation of b proteins or an increase in their quantity if they had already been induced (after detopping). Their presence in healthy or P. tabacina-contaminated tobaccos did not seem to be related to specific resistance genes as they were detected in both susceptible and resistant varieties, whether this heredity was expressed or repressed. Their presence in relatively large quantities in field grown plants did not prevent the mold from causing extensive damage. Their production was inhibited after treatment with the ortherwise very effective metalaxyl fungicide. Polyacrylic acid, the inducer of b proteins, gave no protection against this parasite; in seedlings (Mg2+) it inhibited the expression of the resistance genes and thus favoured development of the parasite. All these results suggest that b proteins are not directly involved in the resistance mechanism to P. tabacina.