Coronavirus infectivity is exquisitely sensitive to pH. For example, the MHV-A59 strain of coronavirus is quite stable at pH 6.0 (acidic) but becomes rapidly and irreversibly inactivated by brief treatment at pH 8.0 (alkaline). Human coronavirus strain 229E is maximally infective at pH 6.0. Infection of cells by murine coronavirus A59 at pH 6.0 (acidic) rather than pH 7.0 (neutral) yields a tenfold increase in the infectivity of the virus.