SUMMARY: Glutaraldehyde is a 5-carbon dialdehyde with a wide antimicrobial spectrum; its effectiveness is markedly increased by buffering, usually with sodium bicarbonate, between pH 7.5 and 8.5. Its properties as an antimicrobial agent and chemosterilizer have been reviewed by Rubbo, Gardner & Webb (1967), and it is used extensively as a fixative in electron microscopy. We suggested (Munton & Russell, 1970) that glutar-aldehyde acted on the wall of Escherichia coli, although other sites of action were also possible. We report here the effects of the dialdehyde on the stabilization of wall-less forms (protoplasts) with particular emphasis on their ability to withstand osmotic shock.