The kinetics of racemization of isoleucine contained in bovine bone fragments has been studied in detail over the temperature range 100° to 148°C. These elevated temperature studies were used to evaluate the rate of racemization at the lower in situ temperatures at which bones are found in nature. At 20°C the half-life for the racemization reaction in bone is 110000 years. The amount of racemization of isoleucine was used to estimate the age of two fossil bones, one from a cave on the Island of Mallorca (Spain) and the other from the core of deep-sea manganese nodule. The calculated ages are in close agreement with the ages determined using radiochemical methods. These preliminary results suggest the isoleucine racemization reaction may provide an important tool for dating of fossil bones which are too old to be datable by radiocarbon. The major limitation of the reaction is that some estimate of the temperature history of the fossil bone must be available.