Basic endochitinases are abundant proteins in Castanea sativa Mill. cotyledons. Three basic chitinases were purified with molecular masses of 25, 26, and 32 kD (Ch1, Ch2, and Ch3) and with isoelectric points between 8 and 9.5. Antibodies raised against Ch1 cross-reacted with Ch2 and Ch3. However, Ch3 showed differences when compared with the other two enzymes, especially in its higher cysteine content. The size, amino acid composition, and N-terminal sequence of Ch1 indicate that it is a class II endochitinase and, therefore, has no cysteine-rich hevein domain. Ch1 inhibits the growth of the fungus Trichoderma viride. The biological role of these endochitinases is discussed.