Hemoglobin-free met-myoglobin preparations from premature infants and normal newborns contain a fetal type of muscle pigment (Mb F) which differs spectroscopically and electrophoretically from adult myoglobin (Mb A). Acid met-Mb F can not be distinguished from fetal acid met-Hb (Hb F) by these technics. Alkaline met-Mb F has, however, a characteristic absorption spectrum which is dissimilar from that of alkaline met-Hb F, and, electrophoretically, it moves faster than the fetal met-Hb compound. Mb F is gradually replaced by the adult muscle pigment within the first 6 months of life. Thus, the period of disappearance of the fetal red cell pigment from the circulation approximates the period of disappearance of the fetal pigment from the muscle tissue.