Gamma-carboxyglutamic acid (I), a recently discovered amino acid in fresh bone matrix, was found in fossil bones as old as 50,000 years B.P. Laboratory and field data demonstrated that the absolute concentration of preserved I is a useful quantitative index of in situ leaching. I was measured in 12 well-dated fossil samples for which racemization data were also available. No significant alteration by in situ leaching was detected. I content, amino acid concentration, and amino acid composition can be routinely measured with the D/L ratio in both the calibration and unknown samples to minimize the possibility of introducing significant errors in the cali-bration step. I was not detectable in the calcified skeletons of six invertebrate species from five phyla. The usefulness of I as an index of leaching history in fossils appears to be limited to verte-brate calcified tissues, e.g., bone, teeth, and otoliths. -- AATA