The primary structure of the embryonic zeta-chains of humans is given. Blood was obtained from a case with hydrops foetalis syndrom due to homozygous alpha-thalassemia 1. The zeta-chains were isolated by high performance liquid chromatography on reversed phase (RP8). The peptides for sequence work were generated by chemical methods (cyanogen bromide cleavage and acid cleavage at the Asp-Pro bond) and enzymatic cleavages with trypsin of unmodified and succinylated zeta-chains. The peptides were separated by high performance liquid chromatography and sequenced by automatic N-terminal degradation procedures. The N-terminal residue of the zeta-chains is blocked. Therefore the sequence of the N-terminal tryptic peptide was determined after incubation with chymotrypsin. The zeta-chains are alpha-type chains and consist of 141 amino acid residues. The alignment of the zeta-chains with the human alpha-chains shows 57 amino acid exchanges: Thus it is evident that there is a greater phylogenetic distance between the alpha type chains than between the beta-type chains.