The primary structure of the hemoglobin from guanaco (Lama guanacoë, Tylopoda) is presented. It could be separated into the chains by CM-cellulose chromatography. The sequences have been determined by automatic Edman degradation with the film technique or gas phase method, using the native chains and the tryptic peptides of the oxidized chains as well as a fragment obtained by acid hydrolysis. Guanaco hemoglobin has identical alpha-chains with alpaca and identical beta-chains with all Lama species with the exception that one guanaco in this study had alanine and serine in the ratio 1:1 in position beta 135 whereas a second individual had alanine only. Since the data suggest that the domesticated species llama and alpaca originate from the guanaco, it seems likely that beta 135Ala is the common form. Guanaco, llama, and alpaca show a comparable high blood oxygen affinity, caused by the substitution beta 2(NA2)His----Asn, as an adaptation to life at high altitude.