The Ca (Oxford) antigen was originally isolated from a malignant neoplasm and with few exceptions was reported to discriminate between malignant and non-malignant neoplasms or normal tissues. Using the Ca 1 antibody we have studied the Ca distribution in 54 lung neoplasms and adjacent non-neoplastic lung tissue. Staining of tumours was very focal and the proportion of positive cells varied from about 50% for adenocarcinomas to less than 1% for oat cell carcinomas, which were often negative. Focal cytoplasmic staining can be seen in all neoplasms, whereas membrane staining is mainly seen in their areas of glandular and squamous differentiation. We found consistently strong membrane staining of alveolar type II pneumocytes in non-neoplastic lung. This staining may be useful in differentiating type II cells from alveolar macrophages which only occasionally showed granular cytoplasmic staining, probably due to phagocytosed Ca. Mucin from tumours and bronchi did not stain but there was consistent staining of alveolar serous exudate suggesting extracellular location of Ca.