Mutants of E. coli which require 5-aminolevulinic acid (5-ALA), the 1st intermediate of heme biosynthesis, do not respond to hemin and porphyrins. The probable explanation of the lack of response is that E. coli may be impermeable to hemin and porphyrins. Mutants are described which responded to hemin and porphyrin and to 5-ALA. Indirect evidence is presented that the mutants were permeable to hemin. The mutants showed other phenotypic changes and resembled some mutants which have changes in the cell envelope.