Particles as large as several µm diam. have been observed occasionally in normal milk and commonly in prepartum and postpartum colostrum. These particles can be dissociated by EDTA and their appearance closely resembles that of normal casein micelles. However, they are often too large to have been completely formed within the Golgi vesicles of mammary epithelium and hence some degree of post-secretory aggregation of caseins is thought to occur. Two possible mechanisms of post-secretory aggregation of caseins are: (1) a continuation of the normal processes of micelle assembly in the alveolus and (2) aggregation as a result of limited proteolysis of the caseins during the time the milk is in the mammary gland. Incubation of milk with fibrinolysin, however, failed to produce aggregation of normal micelles.