The monoclonal antibody HNK-1 identifies a subpopulation of lymphocytes containing almost all of the natural killer (NK) and antibody-dependent killer (K) cell activity. Using an immunoperoxidase technique on frozen tissue sections of human spleen, tonsil and lymph node we have demonstrated that cells identified by this antibody are almost exclusively confined to germinal centres. Comparison with the distribution of B cells, T cells and T cell subsets underlines the distinctive distribution of these cells for which a physiological role in the maturation of antibody affinity during immune responses is suggested.