Alamar Blue, an indicator for metabolic cell function, was evaluated as a fluorescent and as a colorimetric dye in drug sensitivity assays for human pathogenic African trypanosomes, Trypanosoma brucei rhodesiense and T.b. gambiense. The experimental conditions were adjusted to find those where the relationship between trypanosome number and Alamar Blue signal was linear over the widest possible range. Fluorescent signals correlated to trypanosome numbers from 10(4) trypanosomes/ml (T.b. rhodesiense) and 10(5) trypanosomes/ml (T.b. gambiense) up to 2-3 x 10(6) trypanosomes/ml when trypanosomes were incubated for 2 h with 10% Alamar Blue. Trypanocidal activity of common drugs (melarsoprol, DFMO, suramin, pentamidine and diminazene aceturate) was determined employing this assay. The IC50 values obtained were comparable to those obtained with another fluorochrome, BCECF-AM. The Alamar Blue assay can be applied for drug screening, since it is simple, reproducible and economical. The assay can also be used in field sites with less equipped laboratories, because in addition to fluorometric endpoint determination, a colorimetric reading is possible.