The immunohistochemical detection of CD1c antigen is described in mantle zone B-cells of the tonsil, lymph node, and spleen, and also in the marginal zone B-cells of the spleen. CD1c expression was observed in most cases of low-grade, but in only a single case of high-grade, B-cell non-Hodgkin's lymphoma. It was not detected in germinal centre cells, nor in Epstein-Barr virus-transformed or Burkitt's lymphoma B-cell lines. This distribution suggests that CD1c expression may occur preferentially in slowly proliferating B-cell populations and does not support previous suggestions that CD1c is a human equivalent of the mouse thymus leukaemia antigens.