Monoclonal antibody (MAb) 123C3 was raised against a membrane preparation of a small cell lung carcinoma (SCLC) specimen and its reactivity on normal tissues was tested. For the endocrine system, positive tissues included: pituitary and adrenal glands, thyrocytes and C-cells of the thyroid, the parathyroids, testis Leydig cells and pancreatic islets. In bronchioles and intestinal epithelium occasional cells, resembling Kultchitsky and enterochromaffin cells, were also positive. Epithelia like rete testis, mammary epithelium and gastric mucosa were positive in all or a significant proportion of cells. The positive cells in mammary epithelium and gastric mucosa were too numerous to represent the endocrine cells only. Neurons were usually negative or weakly positive. Their supportive cells such as glial, Schwann and ganglionic satellite cells were positive. Mesenchymal cell types, such as smooth muscle cells in most organs, cardiac muscle cells, the pia-arachnoid and ovarian stroma cells were positive, indicating that 123C3 reactivity is not confined to epithelial and neuron-supporting tissues. In Western blots of tumour specimens 123C3 recognized a 29 kDa band in reducing conditions, shifting to approximately 150 kDa in non-reducing conditions. Immunofluorescence on live tissue culture cells demonstrated presence of the antigen on the cell surface.