Crude and purified enzyme preparations from Acrophialophora nainiana and Humicola grisea var. thermoidea were separately incubated with delignified and non delignified samples of eucalyptus kraft pulp at the charges of 1.5, 2.5 and 5 IU/g dry weight pulp. The effect of enzymic treatment on the pulp was evaluated through spectrophotometric and scanning electron microscopy techniques. For the crude enzyme preparation (at charge of 1.5 IU/g dry weight pulp) from H. grisea var. thermoidea the release of chromophores and reducing sugar was observed to be maximal after 3 h of reaction time. In comparison to the control, this release was enhanced by 85, 71, 60, 55 and 78% at 237, 254, 465 and 540 nm, respectively. With A. nainiana enzyme treatment, the highest absorbance values occurred after 4 h and the release of chromophores and reducing sugar was improved by 15, 28, 20, 12 and 40% at 237, 254, 465 and 540 nm, respectively. A defibrillation of the microfibrils was detected for the pulp enzymically treated when compared with the control. The effects observed on the pulp treated with different enzyme preparations were similar regardless of their biological source (A. nainiana or H. grisea var. thermoidea). (C) 2004 Elsevier Ltd. All rights reserved.