A modification of MacConaill's lead-haematoxylin has been found to stain several endocrine cells producing polypeptides and monoamines, particularly A and D cells of the pancreatic islet, thyroid C cells, gastro-intestinal enterochromaffin cells, gastric G and X cells, pituitary ACTH and MSH cells, adrenal medullary cells, and chemoreceptive cells of the carotid body. A careful comparison of the results of this method with those of HCI-basic dye method and of monoamine methods suggested that carboxyl groups of proteins may be the main binding site of lead-haematoxylin. Experiments with various pretreatments of tissue sections support such a hypothesis. The possibility that biogenic amines take also some part in the staining cannot be ruled out.