Deoxyadenosine triphosphate (dATP) is present in adenosine deaminase (ADA)-deficient or ADA-inhibited human red cells and in the red cells of the opossum Didelphis virginiana. In order to investigate the functions of dATP in the red cell, red cells were treated with 2'-deoxycoformycin (dCf), a powerful inhibitor of ADA, and incubated with phosphate, deoxyadenosine and glucose. These red cells in which ATP was almost completely replaced by dATP, had the same shape, lactate production, nucleotide consumption, stability of reduced glutathione, osmotic fragility and cell deformability as red cells containing ATP. Cells merely depleted of ATP showed reduced viability. This indicates that dATP compensates well for the absence of ATP and acts as an energy-transferring molecule to maintain cell viability. These results indicate that the accumulation of dATP or the reduction of ATP is not the cause of the hemolysis observed after dCf administration.