Organic phosphates in human erythrocytes were selectively varied by incubating fresh human erythrocytes in phosphate-buffered saline containing inosine, pyruvate, adenine, and/or adenosine in various concentrations. The deformability of erythrocytes was measured at 24 degrees C with a rheoscope under shear stress of 8-82 dyn/cm2. (1) With increasing 2, 3-DPG (5 approximately 15 mM/l cells), undeformable erythrocytes increased due to the increased mean corpuscular hemoglobin concentration (MCHC). However, these cells became deformable, when the MCHC was reduced by suspending in hypotonic medium. (2) At the same MCHC, the deformability of 2, 3-DPG-enriched erythrocytes was still reduced, compared with that of control erythrocytes, probably due to altered membrane viscoelastic properties. (3) 2, 3-DPG-reduced erythrocytes (2.2 mM/l cells) was not altered in their deformability. (4) Deformability of 2, 3-DPG-enriched erythrocytes was not changed by lowering oxygen tension. (5) Deformability of erythrocytes was not affected by varying intracellular ATP in the range of 0.5 approximately 2.2 mM/l cells (ATP in control cells was 1 mM/l cells). (6) Increment of IMP (approximately 0.9 mM/l cells) and ITP (approximately 0.5 mM/l cells) did not alter the deformability of erythrocytes. (7) Interaction of intracellular organic phosphates with membrane proteins was discussed.