A practical modification of a reducing-sugar method to estimate soil (carboxymethyl) cellulase activity is described and used to compare three non-sterile soils. The original method involves the formation of a ferric-ferrocyanide complex in soil extracts prepared and exposed to substrate in 2 MOLAR acetate buffer. The authors used 67 milliMolar acetate buffer and were able to detect and compare cellulase activities in three diverse soils using undiluted extracts. The values obtained were more reproducible, but in a different range from those of the original method. The use of undiluted soil extracts prepared in less concentrated buffer simplifies and extends the practicality of the assay by: (1) reducing the importance of using matrix-matched standard curves; (2) requiring less sample manipulation and glassware, and (3) preventing the dilution to extinction of enzyme activity. It also improves the usefulness of the method as an indicator of ecological effects (biomass turnover) due to the introduction of nonindigenous microorganisms and chemicals of environmental concern. These factors make the method more competitive with other reducing-sugar assays used to measure cellulase activity in soil. (Copyright (c) 1994 Government of Canada).