An extracellular complex was isolated from the culture filtrate of Ferrobacillus ferrooxidans grown on either iron or sulfur. The complex isolated from the culture filtrate of iron-grown cells was red with an absorption maxima at 500–510 mμ. This complex consisted of approximately 60% non-phosphorus lipid. Ferrous iron was exclusively associated with a lipopolysaccharide–phospholipid fraction of the complex. The sulfur complex unlike the iron complex contained approximately 70% phospholipid and was yellowish. On the basis of hexosamine and heptose values, less lipopolysaccharide appeared to be associated with the phospholipid fraction of the complex from sulfur-grown cells than from the iron-grown cells.