The effect of cadmium ions on the dioxygen affinity, the time-dependent depletion of intracellular polyphosphates, and the elongation of human red blood cells (RBC's) was examined. The incubation of RBC's in the presence of 1 mM Cd2+ at 37 degrees C for more than one hour results in a decrease of the p50 value by 2.5-3.0 mmHg in comparison to controls. The p50 of stripped (phosphate-free) hemoglobin is not affected by the presence of 1 mM Cd2+ (p50 = 4.8 mmHg at pH 7.2 and 37 degrees C). Experiments with RBC cryolysates demonstrate an apparently competitive effect of 2.3-bisphosphoglycerate (DPG) with cadmium ions on the dioxygen affinity. From 31P NMR spectra, 31P T1 relaxation, and 31P T2 relaxation behavior a more direct evidence for DPG-Cd2+ complexation is obtained. 31P NMR spectra of RBC cryolysates also indicate DPG-Cd2+ complexation. The hydrolysis of free polyphosphates in RBC's incubated at 37 degrees C as monitored by 31P NMR spectra can be noticed after a three-hour lag phase (constant polyphosphate level). This lag phase is lengthened from three hours to four hours in the presence of Cd2+ ions. RBC elongation, as a measure of deformability, decreases slightly upon incubation with 1 mM Cd2+.