We studied the pattern of calcification and the expression of osteopontin protein and mRNA by the histiocytes and noninvasive carcinoma cells of 20 breast cancers and the histiocytes and atypical ductal cells of sixteen cases of atypical cystic lobules. Ten breast cancers showed low-grade cribriform carcinoma in situ containing secretory material; the remaining ten cancers displayed high-grade carcinoma in situ with central necrosis characteristic of comedo type carcinoma. Hematoxylin-eosin and Kossa staining revealed calcium hydroxyapatite calcifications in 80% of cribriform type carcinomas, 50% of comedo type carcinomas, and 56% of atypical cystic lobules. Immunohistochemical staining demonstrated osteopontin protein in intraluminal secretory material, necrotic debris, or stroma, and in histiocytes in all the Kossa-positive carcinomas and atypical cystic lobules. In situ hybridization revealed osteopontin mRNA mainly in the histiocytes and especially in those near the calcifications. In two cases, rare carcinoma cells contained osteopontin protein and mRNA. The close relation between hydroxyapatite crystals and osteopontin-producing histiocytes suggests that osteopontin plays a role in the biomineralization that occurs in certain noninvasive breast cancers and atypical cystic lobules. The differences in the morphology of the calcifications and the intraductal contents suggest that the mechanism leading to the osteopontin production might vary depending on the underlying lesion.