Differential binding curve measurements of carbon monoxide and oxygen binding to human hemoglobin F0 under near-physiological conditions (0.1 M NaCl and 15 mM 2,3-diphosphoglyceric acid, pH 7.35, and 37 degrees C) have allowed a detailed description of the binding and linkage between these two gaseous ligands. Comparison with human hemoglobin A0 under identical solution conditions shows that fetal hemoglobin F0 binds oxygen and carbon monoxide with higher affinity than human hemoglobin A0, but with the same cooperativity. Construction of the partition coefficient surface for carbon monoxide and oxygen binding reveals a failure of Haldane's laws for both hemoglobins. Linkage graphs are used to explore the phenomenological properties of the system. The graphs provide a quantitative description of the mechanism of carbon monoxide toxicity on oxygen transport by hemoglobin in vivo and demonstrate striking similarities between the functional properties of fetal and adult hemoglobins.