A number of obligately anaerobic rumen streptococci have been found capable of hydrolysing a mannan, galactomannan, contained in the endosperm of Cyanopsis tetragonaloba. Some of their morphological, cultural and physiological characteristics have been determined The enzyme(s) associated with the breakdown of galactomannan is termed "galactomannanase". It is possessed by some authentic rumen bacteria, and it is also found in significant amounts in raw rumen liquor. Galactomannanase in bacteria-free rumen fluid is postulated to be released into it by the prior growth of micro-organisms attacking the [beta]-1:4 linkage such as cellulose, cellulose derivatives and galactomannan. A survey of a number of non-rumen bacteria revealed that they do not elaborate galactomannanase. Ability to hydrolyse galactomannan seems confined to a specialized group of anaerobic cocci of the rumen. Procedures for the purification of galactomannan and galactomannanase are described.