Bartonella henselae, a facultative intracellular bacterium, has been known as the agent of cat scratch disease, bacillary angiomatosis, peliosis hepatis, endocarditis, and bacteremic syndrome in humans. Bartonella species can cause intraerythrocytic infections and have been isolated from the bloodstream of patients by several methods. It was demonstrated that B. baciffiformis and B. quintana infect human endothelial cells and human erythrocytes and B. henselae infects erythrocytes of cats. The aim of this study was to investigate through transmission electron microscopy whether B. henselae infects mature human erythrocytes. One red blood cell (RBC) unit received an experimentally standard strain of B. henselae. Blood aliquots were collected from the infected unit immediately after inoculation, at 30 min and 1, 5, 10, and 72 h for ultrastructural evaluation. B. henselae was seen adhering to human erythrocytes 10 h after inoculation and inside the erythrocyte after 72 h. This study demonstrates that B. henselae adheres to and invades mature human erythrocytes. The results favor the possibility that erythrocytes can serve as a primary target in Bartonella spp. infections. From this observation, further studies are warranted to prevent Bartonella spp. transfusional transmission.3141794369372