Eighty-three non-Hodgkin lymphomas classified according to the Kiel classification have been studied with regard to surface immunoglobulin (sIg) on cells in suspension and cytoplasmic immunoglobulin (cIg) by the peroxidase anti-peroxidase method (PAP) on formaline-fixed tissue sections. Fifty-six out of 66 examined (i.e. 85%) revealed a monoclonal staining pattern for sIg, whereas 37/70 (53%) gave a monoclonal staining pattern for cIg by PAP. The methods combined gave a monoclonal staining pattern in 73/83, i.e. in 88%, of the biopsies tested. The discrepancies between the two methods were largest in centroblastic/centrocytic and lymphocytic lymphomas. With regard to the light chain staining patterns, complete agreement between the two methods was obtained in the 20 cases that allowed such analysis to be made. This suggests that the specificity of PAP, as carried out in this study with reagents purified by immunoabsorbent techniques, is satisfactory. On a basis of heavy chain isotypes centroblastic/centrocytic, lymphoplasmacytoid, and immunoblastic lymphomas could be divided into distinct immunological subgroups. In four biopsies the sIg heavy chains were mu + delta, whereas mu + gamma chains were detected by PAP. This finding may be relevant to the mu leads to gamma switch known to occur during normal B-cell differentiation. Immunoglobulin inclusions were found in 8 cases--3 belonging to the immunoblastic group, and 5 to the lymphoplasmacytoid group.