Frozen spleen of mice immunized with vaccine from S. sonnei has been homogenized in the isotonic saline solution and centrifuged. That supernatant has been shown to inhibit antibody immune response to thymus-dependent antigens (sheep and rabbit erythrocytes) and hypersensitivity of the delayed type to splenocytes of guinea pig. The supernatant did not inhibit the antibody formation to thymus-independent antigen (bacterial lipopolysaccharide). Immunosuppressive activity of the supernatant disappeared after treatment with anti-immunoglobulin (but not anti-O-antigen) immunosorbent. The spleen of immunized mice is concluded to contain a factor which inhibits T-cell link of the immune system and is of the immunoglobulin nature.