1. A case of paroxysmal nocturnal hemoglobinuria (PNH) has been presented in which the Ham presumptive test for PNH was negative.

2. The basis for this false negative test and the atypical results of the Ham complete acid hemolysis test for PNH appeared to be due to a loss of the in vitro PNH hemolytic activity of the serum associated with a deficiency of properdin.

3. The acidosis accompanying the renal failure in this patient did not appear to accelerate the hemolytic process, for there was no apparent reduction of hemolytic activity in vivo when the acidosis was corrected by the administration of sodium bicarbonate.

4. The autopsy observations suggest that the protracted deposition of massive quantities of iron in the kidneys may play a robe in the not uncommon occurrence of renal disease in PNH.

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