SummaryThe bacteriological formation of 2-butanone and 2-butanol was investigated. The data suggest that 2-butanol is formed in cheese via 2-butanone in 3 steps, each step being carried out specifically by a different microbial species. Suitable species have been isolated from a single cheese. The first step could be carried out (in the present investigation) either by certain starter organisms or by a strain ofPediococcus cerevisiae. This strain formed substantial amounts of 2,3-butylene glycol in milk, presumably from citrate, but could not metabolize this compound further. A second micro-organism, a strain ofLactobacillus plantarum, was isolated which, in Cheddar cheese, could convert 2,3-butylene glycol into 2-butanone. The final reduction of 2-butanone to 2-butanol could be readily carried out in MRS medium by a strain ofL. brevis. Results from cheese made aseptically indicate that 2-butanone is formed in Cheddar cheese only when non-starter organisms are present.