New type of cytotoxicity descriptors based on Cu(II) coordination ability of p-substituted phenols and subsequent O → Cu electron density transfer is introduced. Geometries of phenols and of their Cu(II) complexes are optimized at the DFT level of theory. The amount of electron density transfer is evaluated using the copper charge or the electron density Laplacian at the Cu-O bond critical point. The Cu-phenol interaction energy is a measure of the phenol reactivity. Statistical parameters for the linear dependence of the published cytotoxicity data on the above predictors are much better than the classical ones.