Abstract Objective: The aim of the present study was to determine the erythrocyte CAI and CAN concentrations in fetal blood over a wide gestational range, and compare levels to those found in neonates and adults. Methods: Human erythrocyte CAI and CAII concentrations were measured in fetal (n = 38), neonatal (n =10) and adult (n = 30) blood, using enzyme-linked immunosorbent assays (ELISA). For the measurement of CAII, a new ELISA method was developed. Results and Conclusions: The ELISA method was found to be simple, sensitive, economical, and precise. The normal mean levels of erythrocyte CAI and CAII in adults with standard deviation were 13.68 ± 2.79, and 1.59 $ ̄ 0.21 mg/g Hb, respectively. The corresponding values in cord blood obtained at delivery at 38–40 weeks gestation were 1.20 ± 0.68 and 0.46 ± 0.13 mg/g Hb. The mean CAI/CAII ratio in adults was 8.8 and, in newborns, it was 2.5. The normal mean fetal erythrocyte CAI concentration increased significantly with gestation from 39 μg/g Hb at 20 weeks to 380 μg/g Hb at 38 weeks of gestation. Similarly, the CAII concentration increased from 53 μg/g Hb at 20 weeks to 120 μg/g Hb at 38 weeks of gestation. The CAI to CAII ratio also increased with gestation from 0.9 at 20 weeks to 2.5 at 38 weeks.