In the present immunohistoehemical studies. Leu 7⁺ (HNK‐l⁺. human natural killer and killer) cells were found to occupy preferentially germinal centres of follicles in lymph nodes and tonsils. Leu 7⁺ cells were also present in germinal‐like zones of spleen follicles and in mantle zones of hyperplastie thymus follicles and varied in localization in lymph nodes involved in different types of follicukir centre cell‐derived malignant lymphomas. Most of the Leu 7⁺ cells in the follicles expressed the Leu 3 (helper inducer) marker. Double staining studies of tonsil sheep erythrocyte‐rosetting and peripheral Mood mononuclear cell suspensions showed that two main, mutually complementary, suhpopulalions of Leu 7⁺ cells could he distinguished in hot h cases, namely Leu 7⁺/Leu 4⁺ (subdivided into Leu 2⁺ (suppressor/cytotoxic) and Leu 3⁺) and Leu 7 ⁺/ Leu 4⁻. including mostly cells with OKM 1 (myelomonocytic) characteristics. Thus, in the tonsil cell suspension the cells with Leu 7⁺ Leu 3⁺/OKM 1⁻ imrmmophenotype strongly predominated, whereas among peripheral blood mononuclear cells Leu 7⁺ Leu 2⁺/OKM 1⁻ and Leu 7⁺/OKM 1⁻ iimmunophenotypes were mostly observed.