Embryos from C57BL/6J mice between the gestational ages of 9 and 16 days possess three embryonic haemoglobins EI, EII and EIII, the proportions of which change as a function of gestational age. Component EIj originally present at approx. 65% at day 9, decreases to approx. 20% by day 16 We now report on studies of the developmental pattern of embryonic haemoglobin production in a pure strain of mice (C57BL/6J). The component haemoglobins have been isolated and quantified during early development. An analysis of the 02-binding parameters for each of the isolated components has allowed us to simulate mathematically the anomalous 02-binding curves of the whole-blood system in terms of the contributions of each of the embryonic components. The properties of the individual components and their relative contributions to the whole embryonic erythrocyte system have been rationalized on the basis of the changing physiological demands for°2 encountered by the embryo in its changing intra-uterine environment. Materials and methodsMice of the strain C57BL/6J were obtained from the Animal Laboratory of the Cancer Society of New Zealand and housed under normal conditions. Timed matings were secured by the usual methods (Whitten, 1958). Pregnant female mice were killed and embryos dissected in cold saline solution (0.9% NaCl) as quickly as possible. For embryos of gestational age greater than 11 days it was possible to separate the umbilical blood vessels from the placenta and obtain blood by capillary sampling. Where appropriate, the erythrocytes were separated Vol. 215