Whether interferon has any place in medicine remains to be determined, but it can undoubtedly protect animals against experimental virus infections(1). In previous studies from this laboratory (2, 3) mouse interferon was shown to protect mice against Semliki Forest virus, a highly mouse-virulent arbovirus. It was, however, necessary to inject comparatively large amounts of interferon before or at the same time as the virus, and injections given even 7 hr after the virus had only a marginal effect(3). In most of these experiments usually 100 or more lethal doses of virus per mouse were deliberately used, so that most or all of the interferon-treated mice died, as well as all the controls. Effects of interferon treatments could then be assessed by an efficient method of statistical analysis, based on the survival times of the individual mice. But such large doses of virus are quite unrealistic in terms of the outcome of natural virus infections of animals and man, where an overall mortality of even 30% is an extreme occurrence.