To explore the combined effect of yeast proteins and surfactants on bacterial metabolism. Protein-rich cell-free supernatant from heat-shocked yeast Saccharomyces cerevisiae was combined with certain synthetic surfactants. These blends affected the metabolism of a Polyseed inoculum of aerobic bacteria, accelerating CO2 production and consumption of nutrients from a sterile nutrient broth solution, without a concomitant accumulation of biomass. It is suggested that in the presence of the yeast protein-surfactant complexes, bacterial electron transport is uncoupled from biomass accumulation. The 'uncoupling hypothesis' is supported by experiments with model membranes, in which the same complexes induced proton leak similar to standard chemical uncouplers, such as dinitrophenol, indicating that uncoupling may occur at the stage of generation of the transmembrane pH gradient as the driving force for ATP production. Yeast protein-surfactant complexes behave as uncouplers of oxidative metabolism in bacteria and appear to do so by increasing proton permeability of membranes. Yeast proteins may be of interest as nontoxic, environmentally benign and economically sound agents accelerating oxidative bacterial metabolism while uncoupling it from biomass accumulation. There are actual and potential implications in waste water/soil decontamination, degreasing and other environmental technologies.