Although silicone fibers are among the most compatible with tissue and blood, numerous deposits are observed after their prolonged usage in a capillary membrane oxygenator, even when the blood has been properly heparinized. The scanning electron microscope (SEM) study shows that the morphology of these deposits varies greatly, depending upon the part of the unit from which the sample is taken. The area close to the inlet is the most severely affected. The outlet zone is affected to a lesser degree, and the areas in between are only slightly affected.