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1. Commercial pressed baker's yeast, and cell walls prepared from it, were extracted in various ways and the products examined by a number of techniques, including infrared spectroscopy and electron microscopy. 2. The glucan components of the walls cannot be extracted from intact yeast cells by 3% (w/v) sodium hydroxide at 75 degrees , but at least one-third of the glucan of cell wall preparations is dissolved under these conditions, and more will dissolve after ultrasonic treatment. 3. If intact cells are given a preliminary treatment with acid the wall glucans dissolve in dilute aqueous alkali. 4. Acid conditions as mild as sodium acetate buffer, pH5.0, for 3hr. at 75 degrees are sufficient for this preliminary treatment; the glucan then dissolves in 3% sodium hydroxide at 75 degrees leaving a very small residue, which contains chitin and about 1% of the initial glucan of the wall. Dissolution is hindered by exclusion of air, or by a preliminary reduction with sodium borohydride, suggesting that some degradation of the glucan by alkali is taking place. 5. After treatment with 0.5m-acetic acid for 24hr. at 90 degrees the glucan dissolves slowly at room temperature in 3% sodium hydroxide, or in dimethyl sulphoxide. The extraction with acetic acid removes glycogen and a predominantly beta-(1-->6)-linked glucan (not hitherto recognized as a component of baker's yeast), but none of the beta-(1-->3)-glucan, which remains water-insoluble. 6. Without treatment with acid, the glucan is not significantly soluble in dimethyl sulphoxide, but can be induced to dissolve by ultrasonic treatment. 7. These results are interpreted by postulating the presence of an enclosing membrane, composed of chitin and glucan, that when intact acts as a semipermeable membrane preventing the escape of the alkali- and dimethyl sulphoxide-soluble fraction of the glucan. Mild acid treatments damage this membrane, and ultrasonic and ballistic disintegration disrupt it. 8. Some support for this hypothesis is given by the effects of certain enzyme preparations, which have been found to render a substantial part of the glucan extractable by dimethyl sulphoxide.

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1. Two fungi, Coniothyrium minitans Campbell and Trichoderma viride Pers. ex Fr., were grown on autoclaved crushed sclerotia of the species Sclerotinia sclerotiorum, which they parasitize. 2. In vitro the crude culture filtrates would lyse walls isolated from hyphal cells or the inner pseudoparenchymatous cells of the sclerotia, in which a branched beta-(1-->3)-beta-(1-->6)-glucan, sclerotan, is a major constituent. 3. Chromatographic fractionation of the enzymes in each culture filtrate revealed the presence of several laminarinases, the most active being an exo-beta-(1-->3)-glucanase, known from previous studies to attack sclerotan. Acting alone this brought about a limited degradation of the glucan, but the addition of fractions containing an endo-beta-(1-->3)-glucanase led to almost complete breakdown. A similar synergism between the two enzymes was found in their lytic action on cell walls. 4. When acting alone the endo-beta-(1-->3)-glucanase had a restricted action, the products including a trisaccharide, tentatively identified as 6(2)-beta-glucosyl-laminaribiose. 5. These results are discussed in relation to the structure of the cell walls and of their glucan constituents.

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