Ultrastructural studies were performed on 40 B-cell and 14 T-cell lymphomas of non-Hodgkin's type (NHL). Most B-cell lymphomas were comprised of neoplastic cells with morphologic features compatible with a follicular center cell origin. Dendritic reticulum cells and their desmosome-associated processes, characteristic of germinal centers, were observed in all 11 cases of nodular poorly differentiated lymphocytic lymphoma and in one of two cases of nodular "histiocytic" lymphoma, but were not identified in the lymphomas with a diffuse growth pattern. Desmosomes were observed between dendritic reticulum cells and were not found between lymphoid cells. Large neoplastic cells comprising lymphomas of "histiocytic," mixed lymphocytic "histiocytic," and "undifferentiated" types were characterized ultrastructurally and immunologically as lymphoid cells. Malignant lymphomas of well and moderately well differentiated lymphocytic types (7 cases) revealed B-cell markers, and represented a distinct homogenous group of neoplasms, with electron microscopic features most closely resembling follicular cuff lymphocytes. T-cell malignancies included lymphoblastic lymphomas (3 cases), large cell ("histiocytic") lymphomas (4 cases), lymphoepithelioid cell ("Lennert's") lymphomas (2 cases), mycosis fungoides (3 cases) and diffuse poorly differentiated lymphocytic lymphomas (2 cases). A consistent finding in the T-cell proliferations was the presence of small and/or large lymphoid cells with extremely irregular and/or convoluted nuclei, which occurred in varying proportions and with variable degrees of nuclear complexity. The nuclear irregularity evident in the neoplastic T cells was distinguishable from that observed for lymphoid cells of B-cell lymphomas. In comparing the cytoplasmic features of the T- and B-cell neoplasms ultrastructurally, the only distinguishing feature was the presence of well developed granular endoplasmic reticulum with dilated cisternae, i.e., plasmacytoid features, predictive of a B cell origin.