An immunohistochemical study of non-small cell lung carcinoma using PC10, a monoclonal antibody against PCNA, was performed on tissues routinely processed with formalin fixation and paraffin embedding. The PCNA labelling index and mitotic index were determined from sections of these tissues. Tumours showed a high mean PCNA labelling index of 53.3%. The mean mitotic index was 10.3/1000 cells. Inter-examiner agreement of mitotic counting was good. A linear correlation between the PCNA labelling index and mitotic index was demonstrated (r = 0.71, P less than 0.00001). It is concluded that immunohistochemical nuclear labelling with anti-PCNA on routinely processed tissue is a simple technique for the assessment of proliferation in non-small cell lung carcinoma.