CELLULASES classified as Cx attack soluble derivatives of cellulose, such as carboxymethylcellulose or hydroxyethylcellulose, and also substrates such as alkali or acid swollen cellulose. Highly ordered cellulose as in the cotton fibre is, however, not attacked. The attack on native cellulose is a two-step process and the first of these steps probably involves an enzyme (so-called C1) which attacks the native cellulose in some unknown fashion to produce a reactive cellulose1.