The presence of "monocytoid" cells in toxoplasmic lymphadenitis is a distinctive histopathologic finding, but the exact nature and immunologic properties of these cells have not been characterized clearly. The authors have evaluated the immunohistochemical features of monocytoid cells in fresh-frozen and paraffin-embedded, fixed lymph node material from 21 cases of reactive follicular hyperplasia including suspected cases of toxoplasmic lymphadenitis. This study revealed that monocytoid cells contain (1) the B lymphocyte surface membrane differentiation antigens B-1 and BA-1, and (2) surface membrane-associated immunoglobulin light chains in a polyclonal distribution. No positive reaction was observed when monocytoid cells were studied with monoclonal antibodies specific for peripheral T cells or with antibodies considered to be specific for cells of the monocytic/histiocytic cell line. The results indicate that monocytoid cells, despite their distinct morphologic appearance, have surface marker characteristics of polyclonal B lymphocytes. Therefore, "monocytoid B lymphocytes" would be a more appropriate designation for these cells.