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The development of an automated instrument employing reflectance colorimetry was described. Several models were designed, assembled, and programmed to perform microbial and enzymic tests automatically. Samples were prepared manually or automatically by a Zymate™ II robot. These samples were incubated during the tests to maintain an optimum temperature for reactions and microbial growth. During incubation, color changes of appropriate indicator dyes in the sample/reagent mixtures were measured intermittently, recorded, and compared to previously defined end points. The computer-controlled instrument received data that related time of color changes with the initial numbers of microorganisms or the enzyme activity of the samples. Traditional pH and oxidation/reduction dyes were used. Suitable dyes and media were selected for fast estimation in the different assays studied . Applications of the instrument to evaluate raw milk for the total viable microbial count, abnormality, broad spectrum antibiotics, and coliforms were emphasized. The automated colorimeter system successfully quantitated total and coliform microflora in raw milk. Correlations between reflectance colorimetry and the spiral plate count method were .932 (using .12% TIC as indicator), .922 (using BCP as indicator), and .681 (using .04% TTC as indicator). A coefficient of correlation of .874 was obtained when reflectance colorimetry was compared with coliform numbers on violet red bile agar. The reflectance colorimetry system provided better precision than current reference methods. Preliminary incubation or larger sample volumes were required to estimate low numbers of microflora . Antibiotic residue detection was also evaluated using Lactococcus lactis ssp. cremoris UC 310+ with the automated colorimeter system. The following concentrations (ppb) could be detected: penicillin G:::;; 5, ampicillin ≤ 5, tetracycline ≤ 250 , sulfamethazine ≤ 30, streptomycin ≤ 1000, kanamycin ≤ 500, and chloramphenicol ≤ 500. Abnormal milk could be screened out by measuring the NAGase activity and chloride ion content in milk samples. Both methods had been integrated into the automated colorimeter system. The coefficient of correlation between somatic cell count and the NAGase activity as measured with the colorimeter was .802; a correlation of .792 could be obtained when chloride ion content was measured.