HEPES (N-2-hydroxyethylpiperazine-N-2-ethane sulfonic acid) is a general purpose zwitterionic organic chemical buffering agent which does not bind magnesium, calcium, manganese(II) or copper (II) ions. Buffer strength for cell culture applications is usually in the range of 10 to 25 mM. Studies have indicated that 20 mM HEPES is the most satisfactory concentration of the buffer when both Hanks' and Earle's solutions are used.