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A new minor haemoglobin, L, was isolated from the haemolysates of chicken embryos more than 7 days old. Electrophoresis in denaturing conditions and tryptic peptide maps of the globins show that the beta-like globin of HbL is identical to that of the minor haemoglobin H(beta H) while the alpha-like globin is very similar to that of the adult haemoglobin D (alpha D). HbL completes the description of the map of the minor chicken haemoglobins during embryonic development. In early embryos two minor haemoglobins, M and E, are produced which have the same beta-like globin (epsilon) and differ in their alpha-like globins (alpha D and alpha A, respectively). The same two alpha-like globins will make up the minor haemoglobins of the late embryo, L and H, which differ from HbM and HbE on account of their beta-like globin (beta H). The native tetramers L and M are hard to distinguish from each other. However the constituent epsilon globin can be easily separated from beta H by electrophoresis on polyacrylamide gel in formic acid. With this method we found that the switch of the minor haemoglobins in the blood of chicken embryos starts at the 7th incubation day. The two red cell populations, primitive and definitive, present in the blood of 7-day-old embryos were separated on an albumin gradient and their minor haemoglobins analysed. The haemoglobin couple M/E was found in the primitive erythroid cells whereas the L/H couple was found in the definitive ones. The disappearance of the early haemoglobin couple and its substitution by the late one during embryonic development correlates with the replacement of erythroid lines in the blood.