Specific antibody formation in the spleen was described in mice primed subcutaneously with horseradish peroxidase (HRP) and boosted intravenously with the same antigen. The first specific antibody-forming cells responding to the booster injection were observed after one day in the periarteriolar lymphocyte sheath (PALS). The number and staining intensity of these cells greatly increased subsequently. Between day 3 and 6 after the booster injection a shift was observed in the distribution of antibody-forming cells from the PALS to both the site of entry of the central artery in the PALS and the site where the blood passes via terminal arterioles to the red pulp. Specific antibody-forming cells became highly concentrated in these areas, which constitute the so-called marginal zone bridging channels. After day 6 the number of antibody-forming cells decreased sharply. On the basis of this distribution pattern it was suggested that after subcutaneous priming and intravenous boosting specific antibody-forming cells migrate from the popliteal lymph nodes to the spleen and gradually leave the spleen at later stages of the response. Specific antibody-forming cells did not occur in the germinal centers during any stage of the response.