The Ki-1-antibody was raised against a cell-line of Hodgkin-cells, and reacts with most Hodgkin- and Reed-Sternberg-cells. By use of this antibody a new entity of non-Hodgkin's lymphomas has been identified; the anaplastic large cell lymphoma. We report our experience from studying 16 such tumours diagnosed during the period October 1986 to February 1988. In three cases the anaplastic large cell lymphoma appeared in patients who presented Hodgkin's disease as well. Of 15 cases studied using frozen section immunohistochemistry, ten showed T-, one B-, and one monocyte/macrophagephenotype, whereas three cases presented a mixed T-, B- and monocyte/macrophagephenotype. All cases showed a high proliferative activity as evaluated by presence of activation antigens. In paraffin sections six of 16 cases expressed epithelial membrane antigen (EMA). In 13 cases EMA, leucocyte common antigen (LCA) and Ber-H2 (a Ki-1-antibody) reactivity was compared in paraffin and frozen sections. The EMA reactivity was of nearly equal intensity in paraffin and frozen sections, while the reactivity of LCA and Ber-H2 was significantly reduced in paraffin sections, implying only partial preservation of the antigens. Clinically, most cases were in an advanced (III and IV) stage by the time of diagnosis. Of 14 actively treated patients, nine went into complete and two into partial remission, while three patients did not respond. Six patients are dead, five after less than one year. The coexistence of anaplastic large cell lymphoma and Hodgkin's disease and the common Ki-1 (CD30) antigen expression in these tumours suggest a possible histogenetic relationship.