Bovine casein micelles in pre- and post-partum colostrum have been characterized by both electron microscopy and light scattering. Attempts have been made to relate the changes in average micelle size to the partitioning of Ca and inorganic orthophosphate betwen the colloidal and aqueous phases. Pre-partum colostrum, and to a decreasing extent post-partum colostrum, contain casein micelles several microns in diameter. These giant micelles often contain closed cavities suggesting that they are formed by aggregation of particles comparable in size to normal casein micelles. Electron microscopy of mammary gland alveoli from nearterm pregnant heifers shows clear differences in average size between micelles immediately before and after secretion from Golgi vesicles. It appears that micelles aggregate in the alveolar lumen, possibly as a result of continuing co-precipitation of casein with Ca phosphate.