Molar mixtures of Cbh1 and CelD, result in a combinatorial effort to attack crystalline substrates. Panel A crystalline forms of cellulose, Avicel (AV), cotton cellulose (CC), filter paper (FP) and Panel B, partially hydrated phosphoric acid swollen Avicel (PASC) and soluble carboxymethylcellulose (CMC). 2X and 4X molar excess of Cbh1 and CelD were incubated with various forms of cellulosic substrates and the degree of synergy (ab/a + b) determined . Equimolar Cbh1/CelD mixtures had no combinatorial effect (DS ≤1) on cellulose breakdown. Excess of Cbh1 over CelD resulted in a combinatorial effort (up to 345%) to attack crystalline (A) forms of cellulose while less crystalline forms remained unchanged (B). Excess of CelD over Cbh1 resulted in little or no gain of synergy (DS up to 1.91) for crystalline (A) substrates (except 1:4 FP) and activity was inhibited with less crystalline (B) substrates. For details on activity and degree of synergy, see Additional file 3: Figure S 2 and Methods.