A soil organism, isolated by enrichment culture on unbranched alpha1 leads to 6-mannan backbone from the yeast Saccharomyces cerevisiae, secretes and endo-alpha1 leads to 6-mannanase. We have purified this mannanase to homogeneity and find it to consist of a single polypeptide chain with a molecular weight of about 131,000. The enzyme is unusually heat-stable and appears to be highly extended in shape, possessing very little alpha helicity but with a high proportion of beta structure. The mannanase acts on unbranched alpha1 leads to 6-mannan to produce mannose and alpha1 leads to 6-mannobiose, with the intermediate formation of alpha1 leads to 6-mannooligosaccharides of various sizes. Calcium ion is required for full activity. The smallest substrate is the alpha1 leads to 6-mannotriose, whereas the reduced mannotriose is an inhibitor. The combining site appears to encompass 6 to 8 mannose units.