Spleen sections of mice injected intraperitoneally with sheep red cells (SRBC) were investigated for their ability to haemadsorb. It was found that SRBC adhered to the lymphoid follicles, including the periarteriolar area and the germinal centres, as well as to the red pulp. The degree of haemadsorption could be directly related to tissue pyroninophilia and serum antibody titres. Haemadsorption was inhibited by treatment of tissue sections with anti-IgM and anti-IgG sera and it was demonstrated that the specificity of tissue anti-SRBC activity closely paralleled the class of serum antibody in the course of the primary and the secondary immune response. Treatment of tissue sections with buffered saline (pH 7.2) at 56 °C or with acid buffers resulted in complete elution of haemadsorbing properties from the red pulp, while at the same time lymphoid follicles remained strongly active. The possibility of elution of the antibody-like activity from the red pulp may indicate that this is not produced locally but accumulates at the cell surfaces. The haemadsorption technique made it possible to localize and characterize anti-SRBC activity directly in tissue sections.