The CO2 transport properties of the blood of the hagfish, Myxine glutinosa are markedly different from those previously demonstrated for the lamprey, Petromyzon marinus. As in most other vertebrates, the majority of the CO2 in Myxine blood is transported in the form of plasma bicarbonate. Erythrocyte bicarbonate does have some access to the plasma in the blood of Myxine although the documented bicarbonate movements were not sensitive to the chloride/bicarbonate exchange inhibitor, 4,4-diisothiocyanatostilbene-2,2-disulphonic acid (DIDS). The transmembrane pH gradient in hagfish erythrocytes is also quite different from the gradient previously demonstrated in other agnathans. As in other agnathans, significant differences do occur between the distribution ratios for protons, chloride and bicarbonate ions across the erythrocyte membrane, although the magnitude of these differences is smaller in Myxine. Finally, the absence of pH-dependent changes in erythrocyte water content appears to be a common feature of agnathan erythrocytes.