The number and distribution of Leu-7 + cells, a subset of Natural Killer cells clustered as CD 57 + cells, were studied with an immunoperoxidase technique on reactive lymph nodes (n = 13), malignant lymphomas (n = 60) and Hodgkin's disease (n = 22). Results of paraffin-section immunocytochemistry were compared with those obtained of frozen sections of the same tissues. CD 57 + cells are small lymphocytes mainly located in the germinal centers of reactive lymph nodes and in their malignant counterpart, i.e. the follicular lymphomas. The paragranuloma of Hodgkin's disease, type I nodular, contained high numbers of CD 57+ cells. Nine cases of CD 57+ lymphomas are reported, which were of high grade of malignancy. Their histological subtypes were anaplastic (3 cases) immunoblastic (2 cases), pleomorphic medium and large cell (3 cases), unclassifiable (1 case). Their diverse T-cell (4 cases) or B-cell (2 cases) origin and the various expression of epithelial membrane antigen (EMA) or Kil/Ber-H2 (CD 30) suggest an aberrant CD 57+ phenotype of a subset of large cell lymphomas.