During growth in magnesium (Mg++)-deficient mineral media, Pseudomonas aeruginosa cells synthesise large amounts of H1 outer-membrane protein and are resistant to polymyxins and EDTA. It has been suggested that H1 protein replaces Mg++ as an outer-membrane-stabilising component in Mg++-deprived cells, thereby removing the EDTA target and blocking an adsorption site for polymyxins. Induction of H1 protein synthesis also occurred in P. aeruginosa cells grown in Antibiotic No. 3 Broth (Ab3B), although this medium is not Mg++-deficient. Generally, significant induction of H1 protein did not occur in P. aeruginosa cultures grown in other complex media such as Proteose Peptone and Nutrient Broth, which contained less Mg++ than Ab3B, nor in Isosensitest Broth or Mueller Hinton Broth, which contained higher Mg++ concentrations. H1-protein-induced P. aeruginosa cells from Ab3B cultures, unlike those from Mg++-deficient mineral-broth culture, remained fully sensitive to polymyxin B and, with one exception, to EDTA. It is concluded that induction of H1 protein does not itself confer resistance to polymyxin B, and has no more than a minor role in EDTA resistance. Other cell-wall changes, such as phospholipid modifications and the absence of Mg++, probably account for the resistance of Mg++-deprived cells.