The influence of splenectomy on the antibody response to lipopolysaccharide (LPS: E. coli 0128:B12) was investigated in mice. Splenectomy had little effect on the primary response to the LPS. However, the level of IgG anti-LPS antibodies of splenectomized mice was significantly lower than that of sham-operated mice when the mice were immunized 1, 3, and 7 days after the operation and reimmunized 7 days after the first immunization. There was no significant difference in those immunized 30 days after the operation and reimmunized 7 days later. In mice immunized before splenectomy and reimmunized 30 days after splenectomy, the level of IgG anti-LPS antibodies was low, even in the mice splenectomized 30 days after primary immunization. Our results indicate that splenectomy impairs the antibody response to lipopolysaccharides.