1. The old and the current concepts of the renal handling of extracorpuscular plasma Hgb are briefly reviewed.
2. Studies are presented on the starch gel electrophoretic behavior of Mb alone and Mb in serum.
3. It is suggested that Mb is not bound by Hp, known to bind Hgb, and that this constitutes the reason for the "low renal threshold" of Mb as compared to that of Hgb.