Herpes simplex virus which had been sensitized with IgM antibody was not neutralized by the addition of the purified activated first component of complement. In the presence of an optimum concentration of the first component of complement, however, the sensitized virus was neutralized by the addition of a high concentration of the purified fourth component of complement. Under these conditions, the addition of the purified second and third components of complement failed to enhance virus neutralization. With low concentrations of the fourth component of complement, the addition of the second and third components enhanced virus neutralization.