The normal precursor of the neoplastic cell in Hodgkin's lymphoma is still unknown. Previous reports on the expression of the Hodgkin's cell-associated antigen Ki-1, CD30, on normal cells have been limited to activated lymphocytes. This study demonstrates, however, that cells of the macrophage lineage also are able to express the Ki-1 antigen. The Ki-1 antigen is absent from normal blood monocytes but expressed on up to 85% of macrophage-type cells developed during subsequent in vitro differentiation on Teflon membranes. Unlike other maturation-associated antigens, Ki-1 is found only at late stages of the macrophage primary cultures. Its expression can be enhanced by human interferon-gamma in a fashion similar to that of HLA-DR molecules. In addition, freshly explanted tumor cells from three patients with histopathologic and clinical features consistent with the diagnosis of true histiocytic lymphoma or malignant histiocytosis as well as the permanent cell line SU-DHL-1 could be demonstrated to express the Ki-1 antigen. The phenotype of histiocytic malignancy was further evaluated to be HLA-DR+MAX.26+CD25+-EMA+OKT9+Ki-1+. The results could indicate either that Hodgkin's lymphoma may arise not only from the lymphocyte but also from the macrophage lineage or may emphasize a macrophage involvement in the pathogenesis of this disease.