Amyloid bodies can be found in mammary secretory tissue of various species. These corpora amylacea (CA) have a lamellated structure, contain amyloid fibrils and are predominantly located in the alveolar lumina. The nature of the amyloid was not known, but CA were suggested to originate either from milk casein or mammary alveolar epithelial keratin. In the present report, bovine CA were analyzed histochemically. Furthermore, CA were isolated, analyzed and the amyloid was purified and characterized by amino acid sequencing. CA amyloid appeared to be potassium permanganate sensitive and tryptophan positive, and in this respect different from most other amyloid types except for AA and beta-2 microglobulin amyloid. Gel filtration of purified amyloid fibrils showed a HMW peak and a major 4 kD peak. N-terminal amino acid sequencing showed the amyloid to consist of tryptic-like peptides with an unusually high content of amino acids with bulky side chains. The amyloid protein was identified as derived from alpha-S2-casein. The fragments are of varying length (32, 33 and 45 amino acids), but all start at position 81 of alpha-S2-casein. We have identified a new and unique amyloid protein, and we propose to designate it as A alpha-S2C according to the guidelines for amyloid nomenclature.