Divalent cations are necessary for the activity of neuraminidases from certain sources. Burnet & Stone (1947) and Burnet (1948) reported that Ca2+ was necessary for the activity and stability of a crude preparation of neuraminidase from Vibrio cholerae and this was confirmed by Ada, French & Lind (1961) for the purified V. cholerae enzyme. Likewise Warren & Spearing (1960) reported that the activity of a mammalian neuraminidase was enhanced by Ca2+. However, the isolated neuraminidase of influenza virus showed no dependence upon added Ca2+ until the enzyme was subjected to more stringent desalting on a Sephadex G-200 column (Wilson & Rafelson, 1967); the activity of the enzymes (of A0, A2 and B types) then increased 6-fold when Ca2+ was added. Earlier, Edney (1949) had shown that the enzymic activity of crude preparations of influenza B/lee was enhanced about 4-fold by the addition of Ca2+. In this paper two strains of influenza virus were studied.