Acetaldehyde at above about 0.3 g/l inhibited yeast growth, suggesting that it may contribute to product inhibition in alcohol fermentations when present at high concentrations intracellularly. The toxic effects of acetaldehyde and ethanol were not mutually reinforcing, acetaldehyde appearing to alleviate slightly the effects of ethanol. In support of this, low concentrations of acetaldehyde greatly reduced the lag phase in ethanol-containing medium and increased the specific growth rate.