Ethanol-fixed and Papanicolaou-stained smears of 5 cases of cervical small cell undifferentiated carcinoma and 10 control cases of non-small cell cervical cancer were immunocytochemically examined with monoclonal antibodies (MAbs) against cluster 1 small cell lung cancer (SCLC) antigen, which were found to react with neural cell adhesion molecules (N-CAM). MAbs against cluster 1 SCLC antigen used in this study were NCC-LU-243, NCC-LU-246 and anti-Leu-19. The expression of cluster 1 SCLC antigen was recognized by MAb NCC-LU-243 and NCC-LU-246 in all five cases and by anti-Leu-19 MAb in three. The cluster 1 SCLC antigen was not detected in any of the 10 non-small cell cervical cancers. These results suggest that cluster 1 SCLC antigen is stable and immunocytochemically detectable in Papanicolaou-stained smears and is helpful in the cytologic diagnosis of cervical small cell undifferentiated carcinoma.