Bacteria present in the human gut may produce methylglyoxal--a cytotoxic substance in mammals. This was investigated by studying the activity of methylglyoxal synthase, which produces methylglyoxal from dihydroxyacetone phosphate, and methylglyoxal concentration in growth medium of various bacteria isolated from human faeces. Facultative and strictly anaerobic bacteria isolated from faeces were able to produce methylglyoxal in both defined and complex media. Proteus spp. produced large amounts of methylglyoxal and had the greatest methylglyoxal synthase activity. Supplementing defined medium for facultative anaerobes with glucose 1% w/v did not significantly alter enzyme activity or methylglyoxal production. Inclusion of short chain fatty acids or bile acids in the medium reduced methylglyoxal synthase activity and methylglyoxal production by Proteus spp. None of the organisms examined had amine oxidase activity which could have contributed to methylglyoxal production from aminoacetone.