Intraoperative autotransfusion and topical microfibrillar collagen hemostats have been increasingly used, often simultaneously, in various surgical procedures to minimize intraoperative blood loss and thus reduce the inherent risks of homologous blood transfusion. As moderate amounts of small particles have been shown to pass through filtering devices during intraoperative autotransfusion, concern has been raised over the amount of heterologous collagen fibrils transfused and their effect on the host. We found that 2% of microfibrillar collagen hemostat particles pass through the 20-μm millipore filter contained in the tested autotransfusion device (William Harvey H-4700 cardiotomy reservoir). Using a canine kidney perfusion model, we found multifocal perivascular inflammatory reactions within the renal parenchyma five days after transfusion of filtered autologous blood containing minute amounts of microfibrillar collagen hemostat. The findings demonstrate a strong inflammatory foreign body response to heterologous collagen particles trapped in the microcirculation of the perfused kidneys. It is concluded that despite using filters with a pore size of 20 μm, using intraoperative autotransfusion and microfibrillar collagen hemostats simultaneously creates a potential risk because adverse reactions can be elicited especially within the microvasculature of tissues containing end-arterial circulation.