Volatile compounds produced by Pseudomonas fragi and mixed, natural floras on beef of normal pH (5.5-5.8; glucose greater than 1500 micrograms/g) and high pH (6.3-6.8; glucose less than 10 micrograms/g) included a range of alkyl esters and a number of sulphur-containing compounds including dimethylsulphide but not hydrogen sulphide. Production of the last was a property common to the other Gram-negative organisms tested viz. Hafnia alvei, Enterobacter agglomerans, Serratia liquefaciens, Alteromonas putrefaciens and Aeromonas hydrophila, all of which produced similar off-odours and, with the exception of E. agglomerans, 'greening' on high pH meat. Serratia liquefaciens also produced greening of normal pH meat. Acetoin and diacetyl were major end products of Brochothrix thermosphacta but the related 2,3-butanediol was formed only on normal pH meat. The Enterobacteriaceae produced the same compounds but only on normal pH meat and together with Br. thermosphacta were probable sources of these compounds and of the free and esterified branched-chain alcohols detected in the naturally contaminated samples.