Fifteen p-benzoylphenoxypyridines were initially evaluated for their in vitro activity against rhinoviruses (RV) 1A, 2 and 64 and coxsackie virus (Cox) A21 and for their oral prophylactic and therapeutic activity in Swiss albino mice lethally challenged with Cox A21. One compound, (4-[(5-methylsulfonyl-2-pyridinyl)oxy]phenyl) phenyl methanone, was selected for additional evaluation. These studies showed the compound to possess MIC50 values of less than or equal to 5 micrograms/ml against only 6 of 20 (30.0%) RV serotypes tested. In contrast, the compound was active at concentrations of less than or equal to 5.0 micrograms/ml against 10 of 12 (83.3%) enteroviruses evaluated. In vivo studies showed the compound to significantly protect mice lethally infected with Cox A21 after a single oral dose of 37.5 mg/kg (P less than 0.02) and during a regimen of continuous oral doses of at least 4.7 mg/kg per day (P less than 0.001). Mechanism of action studies indicated that the compound inhibits picornavirus uncoating or some earlier virus-host cell-associated event. Isotopic studies show that (4-[(5-methylsulfonyl-2-pyridinyl)oxy]phenyl) phenyl methanone perturbs HeLa cell macromolecular synthesis at concentrations of as low as 3.12 micrograms/ml. This concentration is only 4-fold higher than the concentration of compound necessary to inhibit Cox A21 RNA synthesis by 90%. This narrow therapeutic ratio limits the potential clinical utility of this compound to all but the most serious picornavirus infections.