Free-field power density has long been used as an index of energy dosing in studies of biological effects of microwave radiation. However, this method of quantifying dose can lead to considerable error if it is used as an index of the rate of energy actually being absorbed by a specimen, because the relative absorption cross sections of different specimens may vary greatly. The integral of absorbed energy is a more meaningful measure of exposure; it can be accomplished by using such sophisticated equipment as twin-well calorimeters, or by using the cruder system of saline-filled phantoms. This paper describes a calorimetric system for the measurement of absorbed energy in laboratory animals and in in vitro samples that can be assembled from such common laboratory equipment as Dewar flasks, magnetic stirrers, foamed polystyrene, and an accurate temperature measuring device. Measurement of absorbed energy by this system for a standard tissue-culture flask are compared to values obtained by electrical measurements, by direct temperature measurements, and by heating and cooling curves. Also, application of the system to measurements of dose in experimental animals is demonstrated.