Hydrolyzes 1,4-beta linked polysaccharide backbones of mannans. Hydrolyzes mannohexaose (M6) preferentially to mannotriose (M4) and less preferentially to mannotetraose (M3), mannopentaose (M5), and mannobiose (M2); hydrolyzes M5 preferentially to M2, and M3, and less preferentially to mannotetraose M4; hydrolyzes M4 preferentially to M3, and less preferentially to mannose (M1), plus very little M2. Does not hydrolyze mannobiose or mannotriose. Does not hydrolyze xlyan, starch, cellulose or galactose. Random hydrolysis of (1->4)-beta-D-mannosidic linkages in mannans, galactomannans and glucomannans. Optimum pH is 5.2. Optimum temperature is about 50 degrees Celsius. Monomer. The disulfide bond between Cys-192 and Cys-259 has not been observed in x-ray crystallograghy (PubMed:16487541). This may be a consequence of the X-ray radiation. Belongs to the glycosyl hydrolase 5 (cellulase A) family.