Investigations of phytochemicals for antiviral activities are assuming greater importance; but little attention has been given to the influence of various reaction parameters. We found that the activities of several known antiviral phytochemicals were profoundly affected by the presence of serum components, but in different ways. Thus, the terthiophene, alpha-terthienyl (alpha-T), was inhibited in a concentration-dependent manner by serum. In the case of a carboxylic acid derivative of alpha-T, the compound appeared to have no antiviral activity at all in the presence of serum, yet in its absence this compound was as effective as alpha-T. In contrast the complex anthraquinone hypericin required a small amount of serum for maximal antiviral activity, although too much was inhibitory. The reactions were also strongly affected by the order of incubation of the components: virus, compound, serum, and light. The antiviral effects were not influenced significantly by temperature, in contrast to a report by other workers, provided the light exposures were controlled. These effects are significant because serum is commonly used in virus assays, and plant extracts often contain polypeptides. Furthermore, when phytochemicals are used in vivo, their effects could be modulated by components of tissues and body fluids.