An acid sialidase partially purified from porcine liver was activated by incubation at 37 °C under acidic pH. This activation was dependent on pH, time and temperature, but not inhibited by amastatin, an inhibitor of aminopeptidase A, in contrast to the case of human placental sialidase. The effects of inorganic anions on the two sialidases from porcine liver and from human placenta were investigated. Among the anions tested, halide ions, especially chloride and bromide ions, markedly enhanced the activation of the two sialidases. However, nitrate, sulfate, sulfite and pyrosulfite ions rarely affected the activation of sialidase from porcine liver, while all of them enhanced the activation of human placental sialidase. The activation of the enzyme from porcine liver was depressed at concentrations of greater than 100 mM of sodium chloride, whereas the enzyme from human placenta was held at maximum activation until 1 M sodium chloride. These results suggest the possibility of the participation of enzyme functions different from that of human placental sialidase in the activation process of sialidase.