A Nile red fluorescent technique to quantify 20 200 mu g ml(-1) of emulsan was developed. Nile red dissolved in DMSO showed an adsorption peak at 552 nm, and emission peak at 636 nm, with molar extinction coefficient of 19,600 cm(-1) M-1. Nile red fluorescence in DMSO was proportionally quenched by emulsan and the quenching was time-dependent. The assay was used to follow the production of emulsan by cultures of Acinetobacter venetianus RAG-1.