An automatic flow injection analysis (FIA) system for on-line determination of esterase activity has been developed. It is based on a colorimetric method using p-nitrophenyl propionate as substrate. The system permits a linear range analysis up to 0.18 U ml(-1), although the range can be extended up to 1 U ml(-1) without external dilution of the sample. The sampling frequency is of 4 samples per h with a relative standard deviation of 0.9%.