The authors examined the immunohistochemical and ultrastructural characteristics of mixed small cell/large cell carcinoma (mixed subtype) of the lung which has been newly categorized as anaplastic small cell lung carcinoma. The 34 cases of small cell lung carcinoma examined consisted of 18 cases of pure small cell carcinoma (pure subtype), 12 cases of the mixed subtype and 4 cases of combined small cell carcinoma. Immunohistochemically, the number of immunoreactive tumor cells for chromogranin A was smaller (P less than 0.05) and that for creatine kinase BB was relatively larger in the mixed subtype. Sialyl LeX-i antigen (SLX), one of the SSEA-1 related carbohydrate antigens, was expressed specifically in the mixed subtype. Ultrastructurally, tumor cells in the mixed subtype had smaller amounts of neurosecretory granules and larger amounts of desmosomes than those in the pure subtype (P less than 0.05, P less than 0.005, respectively). These results suggest that the mixed subtype may show more epithelial than neuroendocrine differentiation and that SLX may be a specific marker for the mixed subtype to be used in pretherapeutic diagnosis.