The red blood cells of the neonatal brushtailed possum exhibit unusually strong cooperativity at high levels of oxygen saturation (n=5.4) which appear to arise from a concentration dependent aggregation of one of the neonatal hemoglobin isoforms. Red blood cells from neonatal pouched young exhibit a Bohr factor of -0.36. Stripped hemolysate is sensitive to added 2,3-bisphosphoglycerate (BPG) (apparent binding constant K=35 micromol L(-1)) and ATP (K=180 micromol L(-1)), but is largely insensitive towards chloride ions. Five isoforms of non-adult hemoglobin were identified using isoelectric focusing. Mass spectrometry indicated that two early isoforms contain alpha chains identical to the adult alpha chain. The remaining three isoforms are composed of identical alpha type and beta type gene products, but differ in their isoelectric points due to differential post-translational modification.