There are two classes of synergism in cellulase mixtures: synergism between endocellulases and exocellulases, and synergism between certain exocellulases. Exocellulases have been defined traditionally as releasing cellobiose from the nonreducing ends of cellulose, but this definition is inadequate to explain exo/exo synergism. Several recent reports indicate that some exocellulases are capable of hydrolyzing cellulose from the reducing end. The existence of two exocellulase classes with different specificities could provide an explanation for exo/exo synergism. In this paper, we report the substrate specificity of three Thermomonospora fusca (E3, E4, and E6) and two Trichoderma reesei (CBH I and CBH II) exocellulases on labeled cellooligosaccharides. We describe a new nonradioactive technique for determining substrate specificity, in which ion-spray mass spectrometry was used to analyze the products of enzymatic digests of cellopentaose labeled with 18O at the reducing end. Exocellulase reactivity was also investigated on cellopentaose labeled at the nonreducing end with 14C, and cellooligosaccharides reduced with NaBH4. The distribution of label in the reaction products supports the existence of two functional classes of exocellulases. One class (containing CBH I, E4, and E6) preferentially cleaves cellooligosaccharides from the reducing end, while the other (containing E3 and CBH II) preferentially cleaves from the nonreducing end. This classification of exocellulases is consistent with exo/exo synergism experiments, and with published cellulase crystallographic data.