A method for introducing biologically active molecules into animal or human cells using an electric current includes suspending the cells and dissolving the biologically active molecules in a buffer solution including HEPES and at least 10 mmol×1−1 magnesium ions (Mg2+), the buffer solution having a buffer capacity of at least 20 mmol×1−1 ×pH−1 at a change in the pH from pH 7 to pH 8 and at a temperature of 25° C., and an ionic strength of at least 200 mmol×1−1. An electric voltage is applied to the suspension.