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A major problem in food technology today is the development of reliable objective tests which indicate changes in foods before deterioration is recognizable by organoleptic means. In meats, particularly beef, changes in acceptability are accompanied by a brownish discoloration. The color change is considered to be caused by conversion of the bright red pigment, oxymyoglobin (MbOn) , to the intermediary dark purplish pigment, reduced myoglobin (Mb), and then to the brown pigment, metmyoglobin (MMb). The reduced myoglobin pigment is found in negligible amounts on meat surfaces or in pigment solution since it is readily oxygenated to oxymyoglobin or oxidized to metmyoglobin.Early work in this laboratory included a study of the behavior of meat pigment extracts, with an investigation of the possibility of developing an objective test for detecting meat deterioration by color change. The spectrophotometric method of Austin and Drabkin (1) modified by Jensen and Urbain (6), which is routinely employed in meat color research, was used for the estimation of metmyoglobin in the presence of oxymyoglobin. This method for estimating the proportion of metmyoglobin in a mixture assumed to contain only two pigments, oxymyoglobin and metmyoglobin, is based on the fact that taken individually, at a standard concentration and pH, each of these two pigments will give a characteristic reproducible absorption curve. At any given wave length the difference between these two theoretical curves may be measured. This difference is considered to be the total change which would occur in a reading a t a given wave length if oxymyoglobin were completely converted to metmyoglobin. The difference between the reading for oxymyoglobin and that for the unknown mixture a t the chosen wave length is considered to represent the partial change the pigment has undergone. Three or four carefully chosen wave lengths which represent the maximum and minimum points of absorbance of oxymyoglobin are used in the estimation of the amount of metmyoglobin present. Estimates made at each of the wave lengths should be comparable and any differences noted should vary in a random manner. In the early studies, however, it was found that a systematic variation existed when the proportion of metmyoglobin was estimated at individual wave lengths. This was true whether the wave lengths 540, 560, 575, and 630 mmc recommended by Austin and Drabkin (1) for methemoglobin estimations or those of 544, 564, 582 mmc, recently reported by Bowen (2) for metmyoglobin were used. The estimates of metmyoglobin made at the wave length 544 and 582 mmc were less variable and consistently higher than those made at 562 mmc. When presumably identical solutions were used, estimates made at 582 mmc were inclosest agreement with each other (8).