A comparison was made of the relative sensitivities of laboratory strain human adenoviruses and enteroviruses, and recently isolated human enteroviruses, to the presence of guanidine hydrochloride in cell culture media. The concentration of guanidine hydrochloride used was 100 microG per ml, corresponding to 59 microG per ml of free guanidine. Representatives of all six human adenovirus groups were unaffected in their replication. Representatives of the human enterovirus groups varied in sensitivity to suppression. Use of guanidine as a media additive successfully facilitated detection of indigenous adenoviruses in concentrates prepared from raw wastewater. It did so through selectively suppressing replication of the more rapidly growing enteroviruses which were likewise present in the same samples. The findings suggest a novel role for antiviral drugs: serving as an adjunct in facilitating selective isolation of specific virus groups when they may be present as part of a mixed viral population.