When studied with double staining techniques HNK-1+ cells include subsets not expressing T cell antigens (A), expressing T8 antigens (B) and expressing T4 antigens (C). Cells with phenotype A are observed as the dominant HNK-1+ population (greater than 50% of all HNK-1+ cells) in the blood from controls and from patients with solid tumours, infectious mononucleosis and sarcoidosis. Cells with phenotype B are always a substantial subset (35% of HNK-1+ cells) in the peripheral blood but in patients with B chronic lymphocytic leukaemia and angioimmunoblastic lymphadenopathy these cells are present in an even higher percentage (greater than 50% of all HNK-1+ cells). This cell subset is the only HNK-1+ population found in the few tumour samples where HNK-1+ cells are identifiable. Apart from these few cases of malignancies, the type A and B subsets are rare in the tissues. In these samples Leu 11+ cells seem to be absent. In contrast, cells with phenotype C are a minor population in the blood but represent most HNK-1+ cells in the germinal centres of lymph nodes and their malignant counterparts in follicular centre cell lymphoma. These HNK-1+, T4+ cells are Leu 11-. These phenotypic characteristics indicate that the most efficient NK cells may represent a circulating and not a tissue seeking population.