Although many in vitro fibrin studies are performed with plasma, in vivo clots and thrombi contain erythrocytes, or red blood cells (RBCs). To determine the effects of RBCs on fibrin clot structure and mechanical properties, we compared plasma clots without RBCs to those prepared with low (2 vol%), intermediate (5-10 vol%), or high (> or =20 vol%) numbers of RBCs. By confocal microscopy, we found that low RBC concentrations had little effect on clot structure. Intermediate RBC concentrations caused heterogeneity in the fiber network with pockets of densely packed fibers alongside regions with few fibers. With high levels of RBCs, fibers arranged more uniformly but loosely around the cells. Scanning electron micrographs demonstrated an uneven distribution of RBCs throughout the clot and a significant increase in fiber diameter upon RBC incorporation. While permeability was not affected by RBC addition, at 20% or higher RBCs, the ratio of viscous modulus (G'') to elastic modulus (G') increased significantly over that of a clot without any RBCs. RBCs triggered variability in the fibrin network structure, individual fiber characteristics, and overall clot viscoelasticity compared to the absence of cells. These results are important for understanding in vivo clots and thrombi.