Abstract: A method for producing haemin proteins by (i) inserting into plant cells one or more nucleic acid molecules that each comprise at least one sequence coding for a protein component of an animal haemin protein capable of reversibly binding oxygen, or for a variant or portion of said protein component, and optionally a sequence coding for a selection agent; (ii) selecting cells containing nucleic acid coding for the protein component of the haemin protein; (iii) optionally propagating the transformed cells either in a culture or by regenerating whole transgenic or chimeric plants; and (iv) recovering and optionally purifying a haemin protein that includes a complex consisting of the protein or proteins coded for by said nucleic acid and at least one iron-containing porphyritic nucleus, or a plurality of such complexes.