Abstract: A method for producing high purity iron oxide nanoparticles using nanoparticle-producing cells, including: a) a pre-growth step that includes amplifying the nanoparticle-producing cell(s) in a pre-growth and/or fed-batch medium/media, and b) a growth step that includes amplifying the nanoparticle-producing cell(s) originating from the pre-growth step in a growth and/or fed-batch medium/media, wherein the pre-growth and/or growth and/or fed-batch medium/media comprise(s), per kilogram or liter of pre-growth and/or growth and/or fed-batch medium/media: i) no more than 0.005 gram of yeast extract, and ii) no more than 0.001 gram of CMR agent selected from boric acid and nitrilotriacetic acid, wherein the fed-batch medium when it is present is a medium that supplements the pre-growth and/or growth medium/media, and wherein more nanoparticles are produced in the growth step than in the pre-growth step.