This study demonstrates that the most sensitive method for the detection and quantitation of cultivable human enteric viruses in water samples after repassage in the MA-104 cell line is the detection of infected cells by the human immune serum globulin--immunoperoxidase (HISG-IP) method recently described by the authors. This immunoperoxidase method is up to 50 times more sensitive than a liquid overlay assay by cytopathic effect in BGM cells. The viral content of waste waters was evaluated with this new methodology. By this method the average viral content of raw sewage (RS) was 900 mpniu/L (most probable number of infectious units per litre), 1056 mpniu/L in primary effluent (PE), and 106 mpniu/L in secondary effluent (SE). With a cytopathic effect assay on BGM cells, values of 85 (RS), 56 (PE), and 2 (SE) mpniu/L were observed, a striking underestimation of the viral content of secondary effluents.