A recombinant E. coli ACV1003 releasing beta -galactosidase by a SOS regulon system when it is exposed to a DNA-damaging compound, has been used to detect endocrine disruptors such as tributyltin (TBT) and triphenyltin (TPT). Maximum response ratio by E. coli ACV 1003 (recA::lacZ) - indicating the maximum ratio of enzyme produced against an environmental toxicant to that produced in the absence of a toxicant - was estimated as 6.3 with 1.0 mug TBT ml(-1) at 37 degreesC, which was considerably higher than those with other strains. Extracellular beta -galactosidase activity was 51 unit ml(-1), which was 5% of that obtained by the conventional Miller's enzyme assay using solvents. Such a low enzyme activity can be rapidly determined, not by the usual time-consuming and tedious enzyme assay, but by an alternative interferometric biosensor. Heavily-doped porous silicon to apply to an interferometer was fabricated by etching to produce a Fabry-Perot fringe pattern, which caused the change in the refractive index of the medium including beta -galactosidase. The change in the effective optical thickness versus beta -galactosidase activity showed a sigmoid increase up to the concentration of 250 unit beta -galactosidase ml(-1).