Tissue kallikrein is well known to liberate the vasoactive peptide kallidin from L-kininogen. Recently it was reported to activate matrix degrading metalloproteinases in vitro and to be present in gastric carcinoma cells. By immunohistochemistry we localized tissue kallikrein in the cytoplasm of ductal breast cancer cells. In addition, two-dimensional Western blotting was used to further characterize its biochemical properties. By this method immunoreactive tissue kallikrein was found to have a molecular mass of 25 kDa and an isoelectric point close to pH 6. Furthermore its presence in human milk could be demonstrated.