Carcinoma-specific antibodies would be a useful tool in immunocytology of serous effusions. We tested the carcinoma-'specific' monoclonal antibody KC4 with cells obtained from pleural effusions evaluated by thoracoscopy and pleural biopsies. KC4 reacted most strongly with carcinoma cells. However, activated mesothelial cells also expressed this marker strongly. Lymphocytes stimulated with phytohemagglutinin were also stained by KC4. Thus KC4 appears to detect a proliferation antigen rather than a carcinoma-specific antigen.