Abstract: An apparatus for treating blood is disclosed which comprises a dialyzer for treating the blood with a treatment liquid and a circulating system which is operative to circulate the treatment liquid through the dializer. The circulating system comprises heating means for heating the treatment liquid to a predetermined treatment temperature and for heating the treatment liquid to a cleaning temperature such that when the treatment liquid is circulated through the circulating system at the cleaning temperature it will effect cleaning thereof. The circulating system further comprises actuating means for actuating the heating means to heat the treatment liquid to the cleaning temperature which means are only operative to actuate the heating means when the circulating system is in communication with the actuating means whereby the actuating means will not be operative when the circulating system is in communication with the dialyzer.