Corpora amylacea were present in variable numbers in the mammary glands of 8 lactating cows but they were not observed in the glands of pregnant or virgin heifers. In primiparous cows they were relatively simple bodies composed of 2--3 concentric layers but in multiparous animals, more complex multilayered structures were also noted. Corpora amylacea were formed in alveoli lined with actively secreting cells. The initial stage of their formation involved the aggregation, fusion and compaction of casein micelles in the alveolar milk. The resulting structure acted as a nucleus for further growth which occurred by the addition of material of micellar origin to its surface. Bodies containing a large proportion of crystalline material were common. Contact of corpora amylacea with the alveolar epithelium often caused flattening or irreversible damage to the secretory cells. The corpora amylacea recovered from milk corresponded in appearance to various stages in their growth seen in the mammary gland. They were usually present in the colostrum of multiparous cows but never in that from primiparous animals. In only 1 cow was the colostrum and milk found to be consistently free of corpora amylacea.