Lymphomas with histologic features indicating a follicular center cell (FCC) origin were analyzed from 26 patients of a group of 45 consecutive non-Hodgkin's lymphoma patinets whose tumors were studied for B- and T-cell characteristics. They were compared with benign, reactive lymphoid tissue from 14 patients. Cell suspensions from biopsy material, blood, or bone marrow were examined for surface Ig and for rosette formation with sheep erythrocytes (E rosettes). Of the 26 patients with FCC lymphomas, 22 had 40% or more Ig-bearing cells; all patients with FCC lymphoma tissues had 25% or less E rosette-forming cells. Cells from most FCC lymphomas of the cleaved type had surfac IgM; those from several FCC lymphomas had both IgM and IgD. Cells from lymphomas of noncleaved cell type had surface IgG or IgA. Light-chain analysis showed that cells from FCC lymphomas bore a predominant light-chain type, which indicated their monoclonal nature. Neoplastic cells from several FCC lymphomas synthesized the surface Ig which they bore. Reactive tissues usually contained fewer Ig-bearing and more E rosette-forming cells than FCC lymphomas; the Ig-bearing cells, with one exception, had a polyclonal distribution. Correlation of histologic and immunologic observations indicates that most lymphomas identified as FCC in origin by light micorscopic criteria mark as B cells with the use of immunologic techniques and that FCC lymphomas are the most common type of non-Hodgkin's lymphoma.