Rapid assays for Escherichia coli have been developed by using the fluorogenic compound 4-methylumbelliferyl β- d-glucuronide (MUG), which is hydrolysed by glucuronidase to yield a fluorescent product that is visible under long-wave u.v. light (Feng and Hartman, 1982; Alvarez, 1984; Koburger and Miller, 1985; Rippey et al., 1987). MUG was incorporated into a Al-broth (Andrews and Presnell, 1972) at a final concentration of 100 mg/l. To each well of a sterile 96-well microtitration plate, 100 μl of Al-MUG medium was added. The plate was then air dried, covered with a sterile tape and stored at 4°C. To enumerate E. coli using the miniaturized multi-tube MPN procedure, 200 μl of diluted or undiluted water was added to each well of a microtiter plate. The number of rows inoculated with each dilution depended on the presumptive contamination level of the samples (Table 1). The plate was incubated for 18 h at 44.5°C and observed in the dark for fluorescence by using a long wave (366 nm) u.v. light source. Raw surface water, wastewater effluents, human and animal faeces were collected for the specificity studies. Fluorescent positive wells were then confirmed for the presence of E. coli by transfer to Schubert broth and incubation at 44.5°C for 24 h. Strains cultivable at 44.5°C, with gas and indole production were considered as E. coli. In addition, gas or indole negative strains were identified using API 20E strips. The Schubert broth test for E. coli confirmed that approx. 88.3% of the fluorescent wells contained E. coli. An 11.7% apparently false-positive rate was also observed. But most interestingly 21 fluorescent wells (5%) that were gas (−) in the Schubert test were confirmed as anaerogenic strains of E. coli with API identification strips. Also 8 fluorescent wells (2%) that were indole (−) in the Schubert test were confirmed as indole (−) strains of E. coli by API strips. So only a 4.8% false-positive rate was observed (Table 3). A study with 5 collaborating laboratories (Nice, Montpellier, Bordeaux, Le Havre and Lille) was conducted in France from June to September 1988 to compare the microtiter plate MPN test with the standard methods: • -3-tube MPN procedure (NPP a 3 tubes), • -membrane filtration (FM), • -Pour Plate Count (IG). Media, time and temperature of incubation, positivity and volumes inoculated are reported in Table 2. Subsurface marine waters were collected aseptically by hand. Testing for E. coli was made before 8 h after collection. Marine waters were collected from the Mediterranean Sea, Atlantic Ocean, English Channel and North Sea. The results indicate that the microtiter plate with MUG had a better specificity than the current methods tested (Table 4). Indeed, 87.3% confirmative rate in the Schubert broth was observed with the fluorogenic assay when 31.7 and 4.10% false-positive rates were observed with FM and IG methods respectively. According to our results, the microtiter plate MUG has been found to be a very consistent and accurate method for the detection and enumeration of E. coli. The performance is equal to 3-tube MPN and FM and better than the IG (Table 5). In conclusion, the microtiter plate method seems to be applicable to routine E. coli enumeration, by incorporating MUG into a selective medium and air drying the medium in a microtitration plate. The ability to detect E. coli is thus improved, and the amount of time and labour required for each analysis is reduced. More precise MPN estimates of E. coli numbers are yielded, and the inoculation and reading of the microtiter plates are easely amenable to automation. By eliminating the battery of biochemical tests, and laborious manipulations required for E. coli identification, and yet providing simplicity, sensitivity and specificity, this method might be an alternative to current estimation of the faecal pollution by the so-called “thermotolerant coliforms” for ill-defined “faecal coliforms”.