Catalase activity in aerobic suspended cell cultures may be followed continuously by using a novel monitoring technique based on a low flow gas meter. Catalase activity was linearly related to dry weight of cells in cultures of Escherichia coli and Saccharomyces cerevisiae, and samples from activated sludge reactors in sewage treatment plants. The lower limits of detection of the monitor as described were 12.5 mg/liter cell dry weight for Escherichia coli, and 73.0 mg/l cell dry weight for Saccharomyces cerevisiae. Catalase activity in soil samples was monitored in a 15-min batch assay by using a similar method and was linearly related to dry weight of a sample of garden soil. Where catalase activity correlates with cell biomass, this rapid assay method may be used to determine biomass levels in soil, and to follow on-line changes in biomass in suspended cultures of aerobically growing microorganisms.