Beneficial effects of biofiltration on acid-base balance have been described, especially in patients showing poor tolerance to standard hemodialysis. This study was designed to standardize the amounts of bicarbonate to be infused for optimal control of the acid-base balance, without the adverse reactions of symptomatic metabolic alkalosis. In three adult patients (body weight greater than 55 kg) a 300 mEq. bicarbonate infusion achieved normal pre- and post-dialysis plasma levels of bicarbonate and normal pre- dialysis pH. Conversely, in three adolescent patients (body weight less than 40 kg) pre- dialysis plasma bicarbonate levels and pre- dialysis pH could not be adequately corrected in spite of increasingly high doses of bicarbonate infused up to a maximum of 240 mEq per treatment. Larger amounts brought on symptoms of metabolic alkalosis.