The cytotoxicity of two kinds of mercurial preservatives, thimerosal and phenylmercuric acetate, was studied using Chang’s human conjunctival epithelia in cell culture. The cultured cells were exposed for 5 s, 2 min and 24 h to each of the two preservatives at various concentrations, obtained by serial dilution. The cytopathic effect and cell desquamation from the wall of culture flask were observed with an inverted microscope and LD50 was calculated by Van der Waerden’s method. The LD50 values of thimerosal were 291.6, 47.4 and 2.2 μg/ml at exposure times of 5 s, 2 min and 24 h, respectively. Those of phenylmercuric acetate were 1,120.2, 227.5 and 2.6 μg/ml at exposure times of 5 s, 2 min and 24 h, respectively. Cytotoxicity can be expressed quantitatively and precisely by LD50. LD50 is lower than the concentrations that are necessary to induce observable morphological changes. The results may help to choose the least toxic concentration of mercurial preservatives for addition to ophthalmic sol