A recombinant putative glycoside hydrolase from Caldicellulosiruptor saccharolyticus was purified with a specific activity of 12 U mg(-1) by heat treatment and His-Trap affinity chromatography, and identified as a single 56 kDa band upon SDS-PAGE. The native enzyme is a dimer with a molecular mass of 112 kDa as determined by gel filtration. The enzyme exhibited its highest activity when debranched arabinan (1,5-alpha-l-arabinan) was used as the substrate, demonstrating that the enzyme was an endo-1,5-alpha-l-arabinanase. The K (m), k (cat), and k (cat)/K (m) values were 18 mg ml(-1), 50 s(-1), and a 2.8 mg ml(-1) s(-1), respectively. Maximum enzyme activity was at pH 6.5 and 75A degrees C. The half-lives of the enzyme at 65, 70 and 75A degrees C were 2440, 254 and 93 h, respectively, indicating that it is the most thermostable of the known endo-1,5-alpha-l-arabinanases.