The immunopathological features of lymphocytic non-Hodgkin lymphomas (NHL), follicular centre cell tumours and "large-cell" NHL are described with particular emphasis on in situ evaluation of cryostat sections using monoclonal antibodies. The immunological phenotypes of tumour cells and the pattern of "reactive" cellular infiltrates with characteristic differences for the various subtypes are summarized. Immunological membrane features of the following lymphocytic NHL of B lymphocyte origin are discussed in detail: B-CLL (most common phenotype of the lymphoma cells: VIB-C5, To-15, Ia, Leu-1), lymphoplasmocytoid NHL (To-15, Ia), B-prolymphocytic leukaemia (VIB-C5, To15, Ia, Leu (+/-), FMC-7(+/-] and hairy-cell leukaemia (HCL: To-15, Ia, FMC-7). NHL of T-lymphocytes detailed in respect of mycosis fungoides and Sézary syndrome (OKT-3, OKT-4, OKT-11), to subtypes of T-CLL with particular emphasis on the "azurophilic" type, to T-zone NHL, and to adult T-cell leukaemia/lymphoma (OKT-3, OKT-4, OKT-11, anti-tac), all of them with the usual immunological phenotype of T-helper (TH) lymphocytes. NHL with a predominance of T-suppressor (TS) lymphocytes include the immunoblastic lymphadenopathy like T-cell lymphoma (OKT-3, OKT-8, OKT-11) and some lymphoepitheloid (Lennert) lymphomas. Immunological features of follicular centre cell tumours are discussed with emphasis on similarities and differences with the normal germinal centre. Tumour cells usually show the following membrane phenotype: centroblastic/centrocytic (cb/cc: To-15, VIL-Al, Ia; meshwork of non-neoplastic R4/23+, To-5 + DRC), centrocytic (VIB-C5, To-15, Leu-1, Ia; R4/23 and To-5 meshwork), centroblastic (cb, To-5, To-15, Ia, VIL-Al +/-). "Large-cell" NHL include the last entity (cb), immunoblastic (B-ib, T-ib) and lymphoblastic (lb) NHL and are discussed in respect to B-ib (Ia, To-15 +/-, VIL-Al +/-), T-lb (WT-1, OKT-6 +/-, OKT-11 +/-), pre-B-lb (VIB-C5, Ia, TdT, VIL-Al +/-) and Burkitt-type NHL (VIB-C5, VIL-Al, Ia, sIg). "Reactive" cellular infiltrates may account for a large fraction of the cellular content of NHL (e.g. the mean TH and TS content amount to about 40% of that seen in normal lymph nodes). In comparison to other NHL B-CLL and cb/cc showed the highest, HCL the lowest number of TH X TS exhibited less typical alterations.(ABSTRACT TRUNCATED AT 400 WORDS)