A Trichoderma reesei cDNA encoding a previously unknown protein with a C-terminal cellulose-binding domain was obtained by complementation screening of a T. reesei cDNA library in a sec1 yeast mutant impaired in protein secretion. The T. reesei protein shows amino acid similarity over its entire length to the Agaricus bisporus cellulose-induced protein CEL1 whose function is not known. These two proteins form a new glycosyl hydrolase family, number 61. Expression of the T. reesei cDNA in yeast showed that it encoded a protein with endoglucanase activity and thus the protein was named EGIV and the corresponding gene egl4. Polyclonal antibodies were prepared against EGIV produced in Escherichia coli and detected a 56-kDa protein in the T. reesei culture supernatant. Northern hybridisation revealed that T. reesei egl4 is regulated in the same manner as other cellulase genes of this fungus.