The HSZP strain of herpes simplex virus type 1 (HSV-1) is defective with respect to the early shutoff of host protein synthesis. However, in superinfection experiments using Vero cells, the HSZP strain was effective, even after neutralization by antibody, at interfering with the early shutoff function of the HSV-1 KOS strain. Evidence was given that the observed interference was not due to exclusion of the KOS by HSZP at the level of adsorption or penetration. The neutralized KOS strain failed to induce early shutoff of host protein synthesis.