Using in-situ immuno- and enzymehistochemical techniques, the phenotype of the neoplastic cells in seven cases of mantle zone lymphoma (MZL) was compared to that in seven cases of nodular poorly-differentiated lymphocytic lymphoma (NPDLL). The neoplastic nodules in MZL consisted of medium-sized lymphoid cells with slightly irregular nuclei and finely dispersed chromatin, expressing monoclonal surface IgM or IgM plus IgD, and displaying membranous alkaline phosphatase (ALP) activity. These cells proliferated around follicular centers that demonstrated a polyclonal pattern of reactivity for both types of light chains and a distorted meshwork of dendritic reticulum cells. The neoplastic nodules in NPDLL consisted of small lymphoid cells with markedly irregular nuclei and coarsely granulated chromatin, expressing monoclonal surface IgM and lacking ALP-activity. These tumor cells also frequently expressed transferrin receptor and common acute lymphoblastic leukemia-antigen (CALLA). The neoplastic nodules showed an undistorted meshwork of dendritic reticulum cells, and were occasionally bordered by remnants of polyclonal lymphocytic coronas. These results confirm the previous suggestion that NPDLL arises from a cell type that is a normal constituent of follicular centers, whereas MZL arises from the lymphocytic corona. The morphological, enzyme- and immunohistochemical features of MZL cells strongly suggest that MZL arises from marginal zone lymphocytes, a subset of corona lymphocytes that expresses ALP-activity, high IgM and low IgD-levels.