The toxic mechanism of La3+ to Escherichia coli is investigated by detecting the concentration change of La3+ in E. coli cells growing in La3+-containing medium. Stimulatory action and inhibitory effect of La3+ in different concentrations can be attributed to the permeability alteration of the cell. Low concentration of La3+ increases the nutrition absorbability of the cells from the cultures as a result of increased cell permeability, and high concentration of La3+ causes the accumulation of La3+ in cells, resulting in the toxic effects on the E. coli cells.