Estimation of tellurium in biological samples by flameless atomic absorption spectrophotometry is hindered by the high volatility of the metal. This necessitates the use of low ashing temperatures which are inadequate to thoroughly ash the samples and thereby reduce interference due to smoke during the atomization stage. The use of platinum as a chemical modifier to thermally stabilize tellurium has, therefore, been explored. Thermal stability of tellurium was dependent on the concentration of platinum; maximum enhancement in stability was achieved at a platinum concentration of 10 microgram/ml or greater, which allowed ashing temperatures to be increased from 400 to 1300 degrees C. A threefold increase in the sensitivity for tellurium determination was also obtained in the presence of platinum. The thermal stability and the sensitivity, however, were susceptible to the presence of organic, inorganic, and biological matrices. This procedure for the determination of tellurium, stabilized probably in the form of an amalgam with platinum, has been used successfully to estimate tissue levels of the metal following administration to mice of a novel tellurium-containing immunostimulant agent. Detection limits in urine, plasma, and tissues were about 50, 5, and 170 ng of tellurium per milliliter or gram, respectively.