An improved immunoenzymatic double labelling method was developed, which simultaneously shows the Ki-1 membrane antigen and the nuclear proliferation associated antigen, defined by monoclonal antibody Ki-67. This new approach permits in situ discrimination of cells that are proliferating or not with a particular membrane antigen. Most Ki-1 positive cells in normal lymphoid tissue also express the Ki-67 nuclear antigen and thus appear to be proliferating. As proliferating cells are more susceptible to malignant transformation than quiescent cells, it is possible that the small number of normal Ki-1 positive cells might, none the less, cause a large proportion of all lymphomas in man.