The release of H2O2 by plant cell suspensions elicited with crude hyphal wall preparations has been studied in a complex of plant genotypes (two cvs of Capsicum annuum and one of C. frutescens) and fungus species (Phytophthora capsici, Ph. parasitica and Verticillium dahliae), representing several combinations of compatibility and both host and nonhost resistance. Production of H2O2 was revealed as peroxidase-dependent and catalase-inhibited fluorescence quenching of an extracellular probe (Pyranine). All the plant genotypes responded to at least one elicitor, but the cell sensitivity showed a great age-dependent variability. Riboflavine and Mn2+ added in the incubation medium acted to some extent as primers for activated cell response, as well as a high Na+ concentration. Cell rest condition, however, was not removed. Some quantitative features of responsive plant/elicitor combinations (dose-response relation and lasting time) have been recorded. The complex PO/H2O2 of elicited cells could perform detectable lignin-like polymerization of an exogenous natural substrate (coniferyl alcohol). The time-course of pyranine oxidation and lignin-like polymer formation could be recorded by adopting a fluorimetric procedure that allowed sequential observations on the same cell sample. In one instance, the cell reaction seemed associated with the in planta host/parasite incompatibility.