The extracellular xylanase activity of 15 actinomycete strains, representing a range of taxa, was examined. Enzyme production was correlated with growth in all cases and product analysis demonstrated that degradation was the result of endoxylanase activity. This was subject to end-product inhibition, probably by xylobiose, but was not inhibited by cellobiose or monomeric sugars. Gel electrophoresis showed that up to six separate endoxylanases were produced but only one was identified in strains that exhibited poor activity. Activity against xylan was found to be specific in that no cross-reactions with endoglucanase activity were detected. The xylan-degrading systems of actinomycetes are clearly complex and non-uniform, although there is some evidence of conservation within Streptomyces.