Infection of NMRI mice with increasing doses of six different strains of herpes simplex virus type 1 (HSV-1) induced increasing levels of neutralizing antibodies. In contrast, three strains of HSV-2, irrespective of the dose, induced only marginal antibody responses. Only strain HG 52 (HSV-2) at high doses of infection stimulated antibody formation. The virus content of some organs in 6- to 8-week-old mice appeared to be lower after HSV-2 than after HSV-1 infection. Application of immune-modulating drugs [silica or poly(I) X poly(C) coupled via L-lysine to CM-cellulose] resulted in little augmentation of antibody formation if compared to HSV-1 infection. Secondary infections with HSV-1 or HSV-2 after a primary dose of HSV-1 were followed by a marked booster response. In contrast, a primary infection with HSV-2 suppressed secondary responses of HSV-1 and HSV-2, thus indicating fundamental differences between the antibody-stimulating potency of HSV-1 and HSV-2 strains.