Because toxicity could result in a decrease in focus formation and be interpreted as evidence of antiviral activity, the inhibitory peptides and their scrambled versions were assessed for cellular toxicity. Confluent monolayers of LLC-MK2 cells used in FFU reduction assays were exposed to increasing concentrations of peptide before measuring mitochondrial reductase activity using an MTT mitochondrial reductase activity assay (Figure 3 ). When we initially performed these assays to exactly mimic the focus forming unit assay by waiting five days after peptide exposure, we saw no evidence of toxicity at any concentration of any peptide (data not shown). However, we found that a shorter postexposure incubation time revealed a subtle toxicity on the part of one of the peptides. Apparently, waiting more than 24 h postexposure gives the cells a chance to recover and conceals this effect. At 24 h post-exposure, DN57opt was found to be mildly toxic to cells at 40 mM (one-way ANOVA with Dunnet's post hoc test, P = 0.0004, N = 18), so only inhibitory data using lower, nontoxic concentrations was considered. Peptides DN57optscr, 