The cytotoxicity of purified condensed cotton bracts tannin towards cultured porcine thoracic aorta and pulmonary artery endothelial cells was examined using (5l)Cr release as a measure of cytotoxicity. A 6 hour exposure to increasing concentrations of tannin resulted in the development of dose dependent lethal cell injury. Thoracic aorta cells were slightly more sensitive to tannin than were pulmonary artery endothelial cells. Although lethal injury was the end result of exposure to tannin, the time course for the development of cytotoxicity was biphasic. No (51)Cr was released during the first three hours of exposure. During this time, however, the cells underwent a striking morphologic change in which they became noticeably demarcated and appeared swollen. In the period between 3 and 6 hours to exposure, (51)Cr release increased almost linearly with time. This lethal injury was accompanied by a second morphologic change in which the cell nuclei became pycnotic. These data indicate that relatively modest concentrations of tannin can cause both lethal and sub-lethal injury to endothelial cells.