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Bacteriocin jenseniin G is detected typically in 50-fold concentrated 10 d cultures of the producer, Propionibacterium thoenii (jensenii) P126 at low concentrations (32-64 AU ml−1). The effect of pH on production was examined by growing the producer in sodium lactate broth, both statically and in stirred fermenters, with or without pH control. Activity was detected in unconcentrated static producer cultures at day 7 through to day 14. Maximal jenseniin G activity (21 AU ml−1) was observed in concentrated supernates at day 9, remaining relatively constant through to day 14. Producer cultures grown in stirred fermenters without pH control yielded detectable activity in 50-fold concentrated supernates at day 3 and maximal activity at day 11. Producer growth in stirred fermenters at controlled pH yielded maximum production at pH 6.40 and maximum activity (160 AU ml−1) at day 13. Jenseniin G activity at pH 6.40 represented a fivefold increase over previous reports. Medium pH was critical to jenseniin G production.