Two endoxylanases produced by C. acetobutylicum ATCC 824 were purified to homogeneity by column chromatography. Xylanase A, which has a molecular weight of 65,000, hydrolyzed larchwood xylan randomly, yielding xylohexaose, xylopentaose, xylotetraose, xylotriose, and xylobiose as end products. Xylanase B, which has a molecular weight of 29,000, also hydrolyzed xylan randomly, giving xylotriose and xylobiose as end products. Xylanase A hydrolyzed carboxymethyl cellulose with a higher specific activity than xylan. It also exhibited high activity on acid-swollen cellulose. Xylanase B showed practically no activity against either cellulose or carboxymethyl cellulose but was able to hydrolyze lichenan with a specific activity similar to that for xylan. Both xylanases had no aryl-beta-xylosidase activity. The smallest oligosaccharides degraded by xylanases A and B were xylohexaose and xylotetraose, respectively. The two xylanases demonstrated similar K(m) and V(max) values but had different pH optima and isoelectric points. Ouchterlony immunodiffusion tests showed that xylanases A and B lacked antigenic similarity.