Estimation of the growth fraction with the use of monoclonal antibody Ki67, which recognizes a nuclear antigen in proliferating cells, was compared with the nucleolar organizer region staining in 95 lung tumours. There was nuclear staining in most tumours; 12 tumours were negative. Cytoplasmic staining was observed in another seven tumours. The small cell carcinoma group had the highest mean Ki67 index (23.75); squamous carcinomas had a mean value of 15.71, adenocarcinomas 10.99, and large cell anaplastic 20.76. Carcinoids had few stained cells. Nucleolar organizer regions were demonstrated by the argyrophilic method (AgNOR). No correlation was found between Ki67 indices and AgNOR scores. Kinetic data obtained by the AgNOR technique were less discriminating in view of the overlap between scores of various groups including carcinoids. We conclude that use of the monoclonal antibody Ki67 is a more reliable method of assessing proliferative activity in lung tumours. This antibody may be effective in identifying slowly proliferating tumours which are less sensitive to chemotherapeutic agents.