Method 1600 describes a membrane filter (MF) procedure for the detection and enumeration of the enterococci bacteria in water. This is a single-step method that is a modification of EPA Method 1106.1 (mE-EIA). Unlike the mE-EIA method, it does not require the transfer of the membrane filter to another medium. The modified medium has a reduced amount of triphenyltetrazolium chloride (TTC) and includes indoxyl Beta-D-glucoside, a chromogenic cellobiose analog used in place of esculin. In this procedure, Beta-glucosidase-positive enterococci produce an insoluble indigo blue complex which diffuses into the surrounding media, forming a blue halo around the colony. Enterococci are commonly found in the feces of humans and other warm-blooded animals. Although some strains are ubiquitous and not related to fecal pollution, the presence of enterococci in water is an indication of fecal pollution and the possible presence of enteric pathogens. The enterococci test is recommended as a measure of ambient fresh and marine recreational water quality. Epidemiological studies have led to the development of criteria which can be used to promulgate recreational water standards based on established relationships between health effects and water quality. The significance of finding enterococcci in recreational fresh or marine water samples is the direct relationship between the density of enterococci and the risk of gastrointestinal illness associated with swimming in the water. Method 1600 was submitted to interlaboratory validation in disinfected wastewater matrices. A summary of method performance from this validation study are provided. A comprehensive evaluation of the study results is presented in the validation study report. Based on these validation study results, Method 1600 is proposed for approval for the analysis of enterococci in disinfected wastewater. Method 1600 for enterococci can be applied to potable, fresh, estuarine, marine, shellfish growing waters, and wastewaters.