Weimberg, Ralph (Northern Regional Research Laboratory, Peoria, Ill.), and William L. Orton. Evidence for an exocellular site for the acid phosphatase of Saccharomyces mellis. J. Bacteriol. 88:1743–1754. 1964.—Evidence is presented which demonstrates an exocellular location for acid phosphatase in Saccharomyces mellis. Derepressed intact cells exhibit acid phosphatase activity. The properties of the system are similar to those shown by the enzyme in cell-free extracts. There is no increase in total activity when cell-free extracts are prepared. Enzymatically active cell walls were prepared by leaching acetone-dried cells of this yeast in dilute acetate buffer (pH 6.5) plus β-mercaptoethanol. The insoluble residue, consisting mainly of cell-wall material and containing the phosphatase, was treated with a variety of hydrolytic enzymes and other chemicals. Only papain and crude snail gut extracts dissociated the enzyme from the particulate fraction in nearly quantitative amounts. The mechanism of release by these two enzymes probably differs. Of all enzymes tested, only the snail gut extract digested the cell walls. By dividing the procedure for making protoplasts of S. mellis into two steps, acid phosphatase may be dissociated from resting cells and recovered as an active soluble enzyme. The first step is to pretreat the cells with a thiol reagent. The second step is to digest the cell wall by enzymes present in crude snail gut extracts. Arsenite must be included in the second step to protect the phosphatase from inactivation. The phosphatase is quantitatively released before the cell becomes osmotically fragile.