Elicitins are 10 kDa holoproteins secreted by Phytophthora fungi, that elicit an incompatible hyper-sensitive reaction, leading to resistance against fungal and bacterial plant pathogens. Comparison of primary sequences of c~-elicitins and fi-elicitins indicated several potential necrotic activity-determining residues. All of the highly necrotic fi-elicitins have a hydrophilic residue (usually lysine) at position 13, whereas in the less necrotic ~-elicitins this residue is replaced by a valine. Here, we report the synthesis and expression of a gene encoding a highly necrotic elicitin, fl-cryptogein, and we show that the substitution of Lys-13 of this recombinant protein by a valine leads to a drastic alteration to the necrotic activity of the recombinant protein.