The ability of small quantities of added acetaldehyde to stimulate growth in environmentally-stressed cultures of Saccharomyces cerevisiae was examined over a broad range of stress conditions. Acetaldehyde addition substantially reduced the lag phase of cultures suddenly inoculated into medium containing inhibitory quantities of low M.W. alcohols and higher fatty acids. For ethanol-stressed cultures, acetaldehyde was effective whether added initially or during fermentation. The effects of acetaldehyde were not universal, only minor stimulation being observed for cultures exposed to heat shock, or experiencing changes in cultivation temperature or pH, despite the occurrence of long lag phases under these conditions. Acetaldehyde strongly inhibited the growth of osmotically-shocked cultures, in contrast to the effects of small quantities of ethanol. The beneficial effects of acetaldehyde appear to be largely confined to cultures exposed to chemical stress especially by agents which disturb membrane structure or function. Acetaldehyde addition has potential practical application in overcoming inhibition in such fermentations.