A sensitive colorimetric bacterial system was developed for the detection of Hg(II) and organomercury compounds. The bioactive species, a recombinant Escherichia coli, produces proportionally elevated levels of the enzyme beta-galactosidase with increasing amounts of Hg. This is due to a reporter plasmid which carriers a Hg(II)-inducible promoter (mer promoter) from the Hg resistance transposon Tn501 regulating the transcription of a promoterless lacZ gene. Additionally, a pMB1 origin of replication without the natural RNA polymerase start site is fused downstream of the mer promoter leading to a Hg(II)-inducible plasmid replication, which results in an improved signal-to-noise ratio. To enhance the sensitivity of this cellular biosensor, the transport proteins for Hg(II) uptake are constitutively produced by a helper plasmid. To enable the detection of organically bound Hg, the Streptomyces lividans organomercurical lyase, an enzyme which catalyses the cleavage of C-Hg-bonds of organomercurial compounds, is also provided by the helper plasmid. Hg(II) and phenylmercuric acetate (PMA) concentrations as low as 5 x 10(-10) M (0.1 ppb) may be detected within a few minutes.