Hemolysis is damage to red blood cells (RBCs), which results in the release of the iron-containing protein hemoglobin into plasma. Here we describe an in vitro assay specifically developed for the analysis of nanoparticle hemolytic properties (see Fig. 1). In this assay, analyte nanoparticles are incubated in blood, and hemoglobin is released by damaged cells and converted to red-colored cyanmethemoglobin by reagents. The nanoparticles and undamaged RBCs are then removed by centrifugation, and the amount of cyanmethemoglobin in the supernatant is measured by spectrophotometry. This measured absorbance is compared to a standard curve to determine the concentration of hemoglobin in the supernatant. This hemoglobin concentration is then compared to that in the supernatant of a blood sample treated with a negative control to obtain the percentage of nanoparticle-induced hemolysis. Fig. 1. Schematic illustration of the steps in this in vitro assay to evaluate nanoparticle hemolytic properties. PFH is plasma-free hemoglobin. CMH is cyanmethemoglobin. TBH is total blood hemoglobin.