Tween surfactants (Tween 20, 40, 60 and 80) stimulated the excretion of protein by Trichoderma reesei QM9414 during growth on cellulose. Tween 60 gave maximal secretion of protein. The presence of Tween decreased the lag time of growth and weakly stimulated growth. Almost the same proportion of cell-wall-bound activity was found in cultures supplemented with Tween for β-glucosidase, acid phosphatase and β-N-acetyl-glucosaminidase. The addition of various Tweens (20, 40, 60 or 80) had only a small effect on the fatty acid composition of the mycelium of T. reesei. Among 5 mutant strains with different secretory capacity, all but one showed stimulation of protein secretion by Tween surfactants. There was no observable correlation between the stimulation of secretion by Tween and the degree of unsaturation of mycelial fatty acids. It is concluded that the stimulation of protein secretion by Tween surfactants does not involve release of surface bound enzymes or changes in membrane fatty acid unsaturation. © 1987.