Studies on living benthic foraminifera commonly involve staining samples with rose Bengal (RB) to distinguish living from dead individuals. Since RB also stains individuals that have died recently (sometimes weeks earlier) and are not fully decayed, standing stocks of foraminiferal communities are usually overestimated. To overcome this bias, we discuss a new viability assay based on the reduction of a tetrazolium salt, MTT (3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide or thiazolyl blue) by living foraminifera. The tetrazolium salt MTT is actively ingested by cells and subsequently converted enzymatically from a yellow, soluble form to a reddish purple crystal. Experiments confirm that living individuals of Ammonia beccarii and Globobulimina turgida convert MTT and become stained within 24 hours. Some dead foraminifers may continue enzymatic activity for several days, but produce a different coloration than that of stained living foraminifers. With the reduced problem of false positives, this assay is an improvement over staining samples with RB whenever a higher accuracy is required (e.g., in short-term laboratory experiments).