The amino acid composition and adsorption properties of the fibril forming protein (FFP), identified previously as a non-hydrolytic component of Trichoderma reseei cellulase enzymes, are described. The amino acid composition was markedly different from the reported endoglucanases (EG) and cellobiohydrolases (CBH) of T. reesei. FFP was bound to Avicel cellulose and acid swollen cellulose rapidly and the equilibrium was reached within 20-30 min. Adsorption on Avicel cellulose was highly dependent on temperature: at lower temperature the rate of adsorption was low although higher amount of FFP bound to cellulose. The energy of activation for binding to cellulose was 6.4 kcal mol(-1). The adsorption equilibrium constant (K-A) and theoretical maximum amount of protein bound (A(max)) were calculated using modified form of the Langmuir equation, and the values were found to be 3.42 x 10(5) 1 mol(-1) and 6.0 mg g(-1) of cellulose, respectively. Binding of FFP was also studied on laboratory prepared cellulose of different crystallinity index (CrI), prepared by ball milling of Whatman filter paper. Cellulose with higher CrI bound higher amount of FFP (3.6 mg g(-1) of cellulose) than cellulose with lower CrI, which was 2.38 mg g(-1) of cellulose. Prolonged incubation of filter paper with FFP resulted in reduction of CrI. Scanning electron micrograph of treated filter paper indicated loosening of the structure of cellulose microfibrils which is hypothesized to result in reduction of CrI.