Thirteen capsules which had formed around breast prostheses were examined, using the technique of splitting the capsule wall. A foreign material in droplet form was present in all the capsules. The foreign material was taken up by phagocytes, and the cytological evidence is that it has cytotoxic activity. It was shown that the material migrates to the vessels in the outer layer of the capsule and may reach the lumen of those vessels. On the basis of the microscopic appearance and the results of infrared spectrum analysis, the foreign material in the capsules was assumed to be silicone.