Enzymes were investigated for their occurrence in the cell wall fraction (4,000 g sediment of the homogenate) of Agaricus bisporus sporocarps. Besides the markers malate dehydrogenase (MalDH), hexokinase (HK) and ATPase, the range of entities studied included gamma -glutamyl transferase (gamma -GT), mannitol dehydrogenase (MDH), phenoloxidase, chitin and beta -1,3-glucan synthases (ChS, beta -GS), chitinase. beta -N-acetyl-hexosaminidase (HexNAc ' ase) and beta -glucanase. Using the extractability in dilute buffer, digitonin and NaCl at high ionic strength as the operational criteria, four categories (I-IV) of enzyme-wall associations could be discerned category I encompasses enzymes which are artefactually present (i.e. contaminants); category II, enzymes that are hydrophobically bound (which may or may not be genuinely wall-associated), III includes enzymes that are ionically bound and IV, enzymes whose bonding to the wall is in all probability covalent. The same enzyme entity may have representatives in more than one category, e.g. ChS and beta -GS (I, II, IV), phenolase (I, II, III, IV), beta -glucanase, chitinase and HexNAc ' ase (I, IV). It is thought that the categorization presented could be of general applicability in fungi as well as in higher plants to specify enzyme-wall associations in a straightforward, comparable manner, thus avoiding some of the ambiguous terms prevailing in the literature, such as "weakly", "strongly- or "tightly" wall bound. The results are discussed in more detail for several of the more economically important enzymes studied.