Several lines of evidence suggest that cellular immunity plays an important role in the defense against herpes simplex virus (HSV) l infections. First, HSV can persist both in vivo and in vitro despite the presence of high concentrations of neutralizing antibody (1). Second, HSV infections are more severe in patients with deficiencies in cellular immunity (2-4). Third, experimental suppression of the cellular immune response makes animals more susceptible to HSV infection (2, 4). The precise way by which cell-mediated immunity operates to protect the host against HSV is far from clear; but recent studies suggest that different immunological mechanisms are required to control different modes of viral spread (5). For example, in the typical lytic infection, HSV can spread by either the extracellular route (Type I spread) or directly from cell-to-contiguous-cell (Type II spread) presumably as a result of cell fusion. Type I spread is readily stopped by antiviral antibody that neutralizes extraceUular virus. Type II spread is not stopped by antiviral antibody because the intracellular location of the virus protects it from neutralization.Little is still known about how the host combats Type II spread, but theoretically, the immune response could stop Type II spread by acting on one or more of three different sites. First, it could act by destroying virus-infected cells. Second, it could break cell-to-cell contact or prevent cell fusion; if this happens the virus would have to travel by the extracellular route to infect adjacent cells and the virus could then be neutralized by antiviral antibody. Third, if the host's immune response leads to the destruction of the surrounding uninfected cells or inhibits viral replication in these cells, the spread of virus would be halted.Evidence that the immune response of the host can directly or indirectly act at each of these sites is beginning to accumulate. It is known that HSV can induce new antigens on the surface of infected cells and that antiviral antibody and complement can react I A bbrevlations used in this paper: C FA, complete Freund's adjuvant; GM, growth medium; HSV, herpes simplex virus; IFA, incomplete Freund's adjuvant; MEM, minimum essential medium; PBS, phosphate-buffered saline; PEC, peritoneal exudate cells; PFU, plaque-forming units; PPD, purified protein derivative; RK, rabbit kidney; VSV, vesicular stomatitis virus.