Protein synthesis elongation factor Tu has been purified from an extreme thermophilic hydrogen oxidizing bacterium Calderobacterium hydrogenophilum. The molecular mass of EF-Tu. GDP is 51,000. The factor is heat stable and loses only 50% of its activity after heating for 5 min at 80 degrees C. Under mild conditions trypsin cleaved EF-Tu. GDP to four main fragments. Only one fragment of Mr = 20,000 had a mobility similar to the trypsin fragment "B" of Escherichia coli EF-Tu. Other peptide fragments of E. coli and C. hydrogenophilum EF-Tu differed in size, but native preparations of both factors are immunologically similar.