Lymph nodes from 13 cases of reactive hyperplasia were examined with four different monoclonal antibodies to B cells. B-1 recognizes an antigen of 30,000 daltons on B cells. CB-2 was prepared with normal spleen and binds to a glycolipid. BA-1 labels surface immunoglobulin-positive cells, but not T lymphocytes or monocytes. B-532 recognizes an antigenic determinant of 45,000 daltons. Using the immunoperoxidase method on frozen sections of reactive lymph nodes, the staining patterns of these four unique antibodies showed dramatic differences. B-1 labeled 80%-90% of the germinal center cells and 10%-50% of the mantle region. Few interfollicular cells were positive. CB-2 stained predominantly in the mantle area (50%-90% positive cells), with moderate staining in the germinal center as well and less than 1% positive cells in the diffuse cortex. BA-1 exhibited predominant labeling in the mantle (50%-90%), with little staining in the germinal center. A large number of cells in the interfollicular, subcapsular, and medullary regions expressed the BA-1 antigen. The B-5 antibody demonstrated intense staining in the follicle (50%-95%). This staining often appeared to be polarized within the germinal center. The mantle zone demonstrated staining of 30%-50% of the cells. The different staining patterns of the B-cell antibodies, as demonstrated by the in situ distribution of positive cells within the lymph node, may reflect stages of development or activation of the B-cell population.