Tristimulus reflectance colorimetry (RC) is a rapid method with which microbial activities can be monitored by the mediation of dye pigmentation changes. Dyes that produce color changes because of shifts of pH and oxidation/reduction (O/R) potential or free amino groups can be used.To estimate total microbial numbers from pure coliform andListeriacultures and from minced beef samples, the appropriate color endpoint values were selected for each sample type (b*=+2 for coliforms,L*=+ 11 forListeria, and a*=+ 4 and a*=+ 6 for minced beef samples) and used to estimate color detection times (CDTs). the regression lines of data between aerobic plate count (APC; conventional and also Spiral System with minced beef samples) and RC were calculated. Reliability of the calibration lines was good; r = 0.90–0.99 for bacterial cultures and r = 0.82 (0.80) for minced beef (a*=+ 4) and r = 0.80 (0.78) for minced beef (a*=+6) when RC was compared with APC (Spiral).The calibration line for minced beef samples (1/10 dilution) could be used to classify samples above and below a certain specified microbial level (e.g., log 6.0 CFU/g) and to marginal samples. Using these scheme with a*=+4 as the color endpoint value, 70% of the tested 40 minced beef samples were correctly classified. the calibration line could also be used to approximate plate count determinations (e.g., CDT of 12 h will result in an estimated count of log 5.9 CFU/g).Results were obtained in shorter time (ranging from 0–15 h forEnterobacter aerogenesandE. coli, from 2–17 h forH. alvei, from 1.5–25 h forListeria monocytogenesATCC 35152 and Scott A, from 6–39 h for slowly growingL. monocytogenesATCC 19919, and from 2–15 h for minced beef, depending on the initial cell concentration) than required by conventional plate counting (24 h for coliforms, 24–48 h forListeria, and 48 h for total counts). Time for further confirmation varies according to bacterial species. the RC technique was sensitive for low levels (as low as 1 CFU/mL) of microbes.Colorimetric microtest plate technology provided in the OmnispecTMBioactivity Monitor System simplifies the analysis, saves labor and materials, and provides