Simultaneous adsorption of poliovirus type 1 and the bacterial viruses f2 and 0X 174 onto electropositive and electronegative membrane filters was studied at pH levels ranging from 3.5 to 8.5. Binding of viruses to the filters was generally better at pH 3.5 as opposed to the higher pH levels. Recovery of adsorbed viruses from the filters was studied utilizing as eluants a beef extract solution, and imidazole-buffered solutions of NaCl, NaCl containing Tween 80, and EDTA containing Tween 80. Efficiency of viral recovery by the combined membrane filter adsorption-elution procedure ranged from 0 to 61 percent. In a comparison of cytopathogenicity, immunofluorescence and in-situ DNA hybridization for the detection of indigenous viruses present in concentrated environmental samples, it was demonstrated that the detection efficiencies of the first two techniques were similar and not statistically different from one another. The efficiency of viral detection by in-situ DNA hybridization averaged 40 percent greater than that achieved by the other two techniques, and this difference was statistically significant.