Lactic acid streptococci have an absolute requirement for magnesium and potassium and failed to grow in milk deionized with IRC 50 resin (sodium form) (Reiter unpublished; see also Fisons, 1958; Reiter, 1956). These findings were confirmed in preliminary experiments using the defined medium of Reiter & Oram (1962). Requirements for other metal ions have now been investigated. Two strains of Streptococcus lactis (ML3 and C10) and one of Streptococcus cremoris (HP) were used. They were maintained as milk cultures, and before use subcultured twice in the synthetic medium containing Ca, Mg, Fe, Zn, Mn, Co, Cu and Mo (cf. Reiter & Oram, 1962) and then finally in the synthetic medium containing only Mg and K (20 ppm.), and Fe (0·01 ppm.).