The elongation factor 1 beta (EF-1 beta), that in eukarya and archaea promotes the replacement of GDP by GTP on the elongation factor 1 alpha x GDP complex, was purified to homogeneity from the thermoacidophilic archaeon Sulfolobus solfataricus (SsEF-1 beta). Its primary structure was established by sequenced Edman degradation of the entire protein or its proteolytic peptides. The molecular weight of SsEF-1 beta was estimated as about 10000 or 20000 under denaturing or native conditions respectively; this finding suggests that the native protein exists as a dimer. The peptide chain of SsEF-1 beta is much shorter than that of its eukaryotic analogues and homology is found only at their C-terminal region; no homology exists between SsEF-1 beta and eubacterial EF-Ts. At 50 degrees C, at a concentration of SsEF-1 beta 5-fold higher than that of SsEF-1 alpha x [3H]GDP the rate of the exchange of [3H]GDP for GTP becomes about 160-fold faster. An analysis of the values of the energetic parameters indicates that in the presence of SsEF-1 beta the GDP/GTP exchange is entropically favoured. At 100 degrees C the half-life of SsEF-1 beta is about 4 h.