THE presence of easily measurable amounts of acid phosphatase in rat brain homogenates was reported by BEAUFAY, BERLEUR and DOYEN (1957). WHITTAKER (1959) isolated different particles from the sucrose homogenate of the mitochondria1 fraction from guinea pigs and from rabbit brain by gradient centrifugation with sucrose of different concentrations. He found the presence of acid phosphatase activity in particles (diameter 0-02-0.3 p) heavier than 0.8 M-sucrose and lighter than 1.2 M-sucrose; later work showed that these particles are derived from pinched-off nerve endings (GRAY and WHITTAKER, 1962). Earlier, RICHTER and HULLIN (1951) found the alkaline phosphatase activity in relatively high concentration in the cell nuclei of human brain cortex. Histological evidence of phosphatase and nucleotidase activities in brain tissue is available (NAIDOO and PRATT, 1951, 1953; NAIDOO, 1962). The above findings indicate that both acid and alkalinephosphatases are present in brain tissue. Recently, in the course of an investigation on the purification and mechanism of action, of ribonuclease from E. coli ribosomes, SPAHR and HOLLINGSWORTH (1961) found an acid phosphomonoesterase activity in the ribosomes; it has been separated from ribonuclease activity and some of its properties have been studied. In a preliminary note DATTA and GHOSH (1962) reported the presence of a phosphomonoesterase activity in ribosomes isolated from goat brain cortex tissue. In the present communication the properties of the alkaline phosphomonoesterase of goat brain cortex ribosomes and its activity towards nucleotides, sugar and other phosphate esters as substrates have been described.