A solid-state, diffuse reflectance-based fiberoptic sensor is described for quantifying ammonia. This sensor is constructed by immobilizing chlorophenol red, a weak acid chromophoric indicator dye, in a microporous polypropylene membrane. A flow-injection analysis system is used to carry a 10-mu L aliquot of the sample across the treated membrane. Ammonia in the sample diffuses through the air-filled pores within the membrane structure before reacting with the indicator dye. A reversible acid-base reaction between ammonia and chlorophenol red results in a measurable change in the reflectance at 560 nm. Response characteristics include a peak response within 20 s, a limit of detection of 0.2 +/- 0.1 mid ammonia, and a dynamic range of up to 60 mM ammonia. The analytical utility of this sensor is demonstrated by measuring ammonia levels in 48 individual samples collected during the growth of PC3 human prostate cancer cells in a typical serum-containing growth medium. Accuracy of the proposed sensor is verified by a comparison of results from this sensor to those from a conventional enzyme assay.