1. Penicillium spec., a mould isolated from rotting leaves, is able to grow on a culture medium with cutin as the only carbon source. Hence the fungus contains enzymes which are able to destroy cutin. 2. While the mould-enzymes were acting upon the cutin, the resulting variations of the nutritional substrate were followed with certain intervals as well by roentgen microscopical examinations as by replica technique; the phenomenons of breakdown could be demonstrated by means of photographs. 3. The change in mechanical properties caused by the enzymes of the fungus are also apparent maCorscopically, moreover the insoluble cutin is converted into a form that is easily soluble in diluted warm alkali. 4. By means of Thunberg technique in water extracts of Penicillium spec, a dehydrogenase could be demonstrated which is able to dehydrogenate fatty acids obtained by chemical hydrolysis of cutin. Some properties of these dehydrogenase were investigated. 5. Using the dehydrogenase reaction as an assay, in extracts of Penicillium spec, with phosphate buffer the presence of an enzyme could be demonstrated that makes cutin readily attackable by the dehydrogenase by liberating free fatty acids from the polymere compound. The preliminary term “cutinase” is proposed for this enzyme. 6. Some properties of the “cutinase” were determined under aerob and anaerob conditions and the stability of the enzyme against dialysis as well as the possibility of precipitation with ammonium sulfate were established. There are indications that preferably monohydroxy stearic acids are used as a substrate by this enzyme. 7. Some results point to the fact that in buffered extracts of Penicillium spec, several enzymes are present taking part in the breakdown of cutin; probably “cutinase” consists of more than one enzyme. 8. Low “cutinase” activity could be demonstrated also in Fusarium moniliforme and higher activity in a yeast, Rhodotorula glutinis. Herrn Professor Dr. H. F. Linskens (Nijmegen) danke ich fur den Hinweis auf das Problem, anregende Diskussionen und die standige Fbrderung meiner Arbeit. Ebenso danke ich Herrn Professor Dr. W. Franke (Koln) fur wertvolle experimentelle Vorschlage und Literaturhinweise, sowie die freundliche Uberlassung von Oxyfettsaure-Praparaten. Herrn M. M. A. Sassen danke ich fur die Isolicrung dcs Penicilliumstammes. Fur experimentelle Hilfe danke ich Fraulein. I. van den Brand, fur die Rontgenaufnahmen Herrn M. van den Donk und fur die Ausfiihrung der fotografischen Arbeiten Herrn H. J. M. Spruit.