To evaluate the clinical significance of monoclonal antibody against human 
pulmonary surfactant apoprotein (S-AP), surgically resected lung cancer from 122 
patients was studied. Paraffin embedded tissues were used for the 
immunohistochemical study by the avidin-biotin-peroxidase complex method. The 
results were as follows. 1. Adenocarcinoma showed highest immunoreactivity for 
S-AP compared to the other histologic types. Among subtypes of adenocarcinoma, 
type II alveolar epithelial type, clara cell type and mixed type of these two 
types were strongly positive (100%, 77.8% and 66.7% respectively). These results 
indicate that this antibody may be a good marker for the subtyping of 
adenocarcinoma. 2. There were some positive cases in other histologic types 
especially in peripheral type of squamous cell carcinoma. These findings suggest 
that this antibody was useful for the histological differentiation of lung 
cancer. 3. As to the immunohistochemical reactivity there was a good correlation 
between tissue and cytological specimens, which indicate cytological studies may 
be adequate for this kind of histopathological studies. 4. In our study, there 
were no patients with S-AP positive carcinomas other than patients with lung 
cancer. These results indicate that this antibody could be used for the 
differential diagnosis between primary and secondary lung cancer.