The constitutive expression of basic peroxidase isoenzymes in the Plasmopara viticola-resistant (Vitis vinifera × Vitis rupestris) × Vitis riparia crossing and in the P. viticola-susceptible V. vinifera parent species was studied. The results illustrate that both leaves and stems of the (V. vinifera × V. rupestris) × V. riparia crossing showed the differential expression of a basic peroxidase isoenzyme B3 (pl = 8.9), this being almost completely absent from the P. viticola-susceptible V. vinifera parent species. To test whether the basic peroxidase isoenzyme B3 may be considered as a molecular marker of disease resistance in Vitis spp., suspension cell cultures derived from the P. viticola-susceptible V. vinifera parent species were treated with an elicitor (cellulase Onoztika R-10) from the soil fungus Trichoderma viride, a specific and well-known elicitor of disease resistance reactions in grapevines. The results showed that treatment with the elicitor induces, simultaneously with the activation of the disease resistance mechanism, the appearance of B3 in the cell cultures. These results suggest that the basic peroxidase isoenzyme B3 may be considered as a marker of disease resistance in Vitis species since it is present in the P. viticola-resistant (V. vinifera × V. rupestris) × V. riparia hybrid and is induced by the elicitor Onozuka R-10 in cell cultures of the P. viticola-susceptible Vitis vinifera parent species. This conclusion is supported by the presence of this isoenzyme in other resistant and its absence in other susceptible Vitis spp.