A population of B cells that regulates the immune response was demonstrated in splenic mononuclear cells (SMNC) from mice infected with herpes simplex virus type 1 (HSV). SMNC, obtained from mice 3 days after HSV infection at a dose of 100 LD50, exhibited a reduction in the proliferative response of naive SMNC stimulated with various lectins or allogeneic lymphocytes in a 5-day mixed lymphocyte reaction. Cocultivation of naive SMNC with phagocytic cell-free SMNC (M phi-MNC) from infected mice resulted in the inhibition of lymphocytic blast transformation stimulated with various lectins. These cells were characterized as nylon-wool adherent cells that were eliminated by treatment with anti-Ig antiserum, but not anti-Thy 1.2 monoclonal antibody or anti-asialo GM1 antiserum, followed by complement treatment. In addition, the suppressor cell activity was not demonstrated in M phi-MNC obtained from HSV-infected CBA/CaHN-xid/J mice, which contain a congenital B-cell deficiency. These results suggest that, in addition to suppressor T cells, a population of B cells, which can inhibit lymphocyte proliferations upon stimulation with lectins and alloantigens, might be generated in spleens of mice following HSV infections.