RMI 15,731 (1-[5-tetradecyloxy-2-furanyl]-ethanone) is a new antiviral compound with activity specific for rhinoviruses. Virus synthesis in R-HeLa cells was susceptible to low concentrations (0.25 /g/ml) of the compound. RMI 15 by the plaque method on R-HeLa cell sheets (80% confluent) contained in 35-mm petri dishes. Each assay was performed in duplicate. A 0.1-mi amount of virus was adsorbed to the cul layer for 90 min at 35°C; 4 ml of overlay medium (0.5% Methocel in BME) containing added magnesium (2) was used on each dish after the adsorption period. Infected cells were incubated for 48 h at 35°C before fixation with Formalin and crystal violet staining.Measurement of cytotoxicity. The toxic effect of RMI 15,731 was assessed by two methods: (i) measurement of cellular protein concentration by the Lowry method as modified by Oyama and Eagle (3) (cell sheets were solubilized in 0.4 N NaOH after washing with saline [3]); and (ii) measurement of total cell number per culture after trypsinization. In all cases duplicate cultures were examined in each experiment at each concentration of compound.Measurement of RMI 15,731 binding to cells. Cell cultures in 35-mm petri dishes were exposed to different concentrations of carbonyl-'4C-labeled RMI 15,731 in 1 ml of BME at 35°C. At various times the medium was removed from duplicate cultures, and the cell sheets were washed three times with phosphatebuffered saline, pH 7.2. The cells in each culture were dissolved in 1 ml of hyamine hydroxide (Sigma Chemical Co., St. Louis, Mo.) and counted by liquid scintillation spectrometry in a toluene-based fluid (4.2 g of 2,5-diphenyloxazole and 52 mg of phenyloxazolylphenyloxazolyl-phenyl per liter).Chemicals. RMI 15,731 is a water-insoluble powder. This compound homogenized into aqueous media was an effective antirhinovirus compound. We found,