Phanerochaete chrysosporium was used for continuous monitoring of cellu- lase action on microcrystalline cellulose (Avicel). Two protocols are described, the parameter monitored being either the decline in electrode potential as ferricyanide is reduced or consumption of dioxygen. Most experiments used a commercial cellulase preparation from Trichoderma reesei and ferricyanide as acceptor. Within 1 min of an addition of cellulase, ferricyanide reduction reached a steady rate. This was converted into a rate of production of substrate for cellobiose oxidase, in Ixmol" m i n -~. Experiments were conducted either with a constant concentration of cellulase and increasing Avicel, or with constant Avicel and increasing cellulase. Kinetic analysis of the experiments with constant cellulase indicated a Km of 4.8-+ 1.0 (g cellulose). 1 -i , which was close to the value predicted from binding studies. The specific activity of the cellulase was measured as 375 + 25 gmol. (g cellulase)-~. m i n -~ in experiments with a high cellulose concentration, but was less than half this value when the cellulose was saturated with cellulase. The maximal rate of cellulose degradation was 9.6 + 1.3 Ixmol. (g cellulose) -1. min -1.