Studies of the growth of Pseudomonas fluorescens type S in acidified peptone nutrient broth supplemented with potassium dioxalatoberyllate show that the inhibitory action of beryllium on the lag phase can be strongly counteracted by an increase in the iron content of the medium. In terms of relative concentrations, Fe3+ is up to 250 times more effective than Mg2+ as an antagonist of beryllium under the conditions employed. Conversely, evidence of the effect of beryllium on iron-dependent constituents of the cell is provided by the fact that cultures of Pseudomonas fluorescens adapted to relatively high concentrations of beryllium show a marked decrease in cytochrome c content.