A newly isolated strain of the fungus, Mucor circinelloides (NRRL 26519), when grown on lactose, cellobiose, or Sigmacell 50 produces complete cellulase (endoglucanase, cellobiohydrolase, and beta-glucosidase) system. The extracellular endoglucanase (EG) was purified to homogeneity from the culture supernatant by ethanol precipitation (75%, v/v), CM Bio-Gel A column chromatography, and Bio-Gel A-0.5 m gel filtration. The purified EG (specific activity 43.33 U/mg protein) was a monomeric protein with a molecular weight of 27000. The optimum temperature and pH for the action of the enzyme were at 55 degreesC and 4.0-6.0, respectively. The purified enzyme was fully stable at pH 4.0-7.0 and temperature up to 60 degreesC. It hydrolysed carboxymethyl cellulose and insoluble cellulose substrates (Avicel, Solka-floc, and Sigmacell 50) to soluble cellodextrins. No glucose, cellobiose, and short chain cellooligosaccarides were formed from these substrates. The purified EG could not degrade oat spelt xylan and larch wood xylan. It bound to Avicell, Solka-floc, and Sigmacell 50 at pH 5.0 and the bound enzyme was released by changing the pH to 8.0. The enzyme activity was enhanced by 27 +/- 5 and 44 +/- 14% by the addition of 5 mM MgCl2 and 0.5 mM CoCl2, respectively, to the reaction mixture. Comparative properties of this enzyme with other fungal EGs are presented. Published by Elsevier Ltd.