One-week-old mice were protected against a uniformly lethal herpes simplex virus (HSV) infection by IL-2 alone, but especially by the addition of human mononuclear cells (MC) plus IL-2. The dose response of IL-2 was biphasic. The addition of MC from cord blood did not enhance IL-2-mediated survival. Because the effect of IL-2 alone, or IL-2 plus MC, was ablated by anti-IFN-gamma and human neonates have an IFN-gamma production defect, the protective effect of MC plus human IFN-gamma (HuIFN-gamma) was tested. MC from adults cultured for 5 days in HuIFN-gamma afforded protection. At least 1 x 10(6) HuIFN-gamma-treated MC were required with increasing survival to 1 x 10(7) MC. The effector cell activity was ablated by adherence, silica, L-leucine methyl ester treatment or treatment with Leu-M3 plus C (all macrophage markers), and OKT4 plus C treatment (CD4 marker). Use of Leu-11, Leu-7, OKT3, or OKT8 plus C did not inhibit protection and excluded NK or T cell participation. In addition to survival, the ability to produce anti-HSV antibody was reconstituted. For the first time protection was afforded by human cord blood MC after treatment with HuIFN in vitro. We have identified an IFN-gamma-driven protection system against murine neonatal HSV infection mediated by human adult- or cord blood-derived CD4-positive macrophages. Protection is associated with enhanced effector cell function and reconstitution of the neonatal antibody production defect.