Type 6 streptococcal M protein produced by E. coli bearing plasmid pJRS42.13 (ColiM6) accumulates in the periplasmic space of this new host. No immunoreactive M protein was found either on the surface of the organism or in the culture medium. The ColiM6 protein was purified from the periplasm and the final preparation consisted of three protein bands of apparent molecular weight 55,000, 57,000, and 59,000. These three bands were identical in migration in SDS PAGE to that of the M protein present in freshly prepared crude periplasm. The amino acid composition of the ColiM6 protein was nearly identical to that of M protein isolated from streptococci with phage lysin (LysM6). Furthermore, except for the amino terminal residue of the LysM6 molecule, the amino terminal sequence of the ColiM6 molecule was identical to those of both LysM6 and M protein released from the streptococcus by limited peptic digestion (PepM6). These results reveal that the molecule produced in the E. coli and transported into the periplasm may be the complete M protein as it exists on the streptococcus. The results also indicate that the systems that process M protein for transport through the cytoplasmic membrane are similar in the streptococcus and E. coli. The purified ColiM6 protein was able to remove opsonic antibodies from both human and rabbit serum, as well as to stimulate the production of opsonic antibodies in rabbits, indicating that the immunodeterminants on this molecule are the same as those found on streptococcal-derived M molecules.