Sixty-six formalin fixed, paraffin-embedded reactive and neoplastic lymph node biopsies were examined by the immunoperoxidase (PAP) procedure to localize toxoplasma antigen. 13:16 (81.25%) of the histologically suggestive toxoplasmic lymphadenitis and 4:8 (50%) of Hodgkin's lymphoma with histiocytic (epitheloid) cell infiltrates were positive. All other reactive and neoplastic biopsies were negative. Our data prove the usefulness of the PAP procedure as a specific and sensitive test to confirm the diagnosis of histologically suspicious toxoplasmosis. This study, however, is the first of its kind that demonstrates immunohistologically the association of toxoplasmosis with Hodgkin's lymphoma in the same lymph nodes. It also calls in question the nature of the histiocytes in lymphoid neoplasms and suggests the possibility that, at least in some lymphomas especially Hodgkin's, the histiocystic component may be due to the association of other conditions which are worth of further investigation.