We describe an innovative microwave instrument, designed in collaboration with and owned by Raytheon Company. The instrument permits the manipulation of biological specimens in their fluid milieu during the actual period of rapid tissue fixation. The specimen chamber is designed for sample containers up to 1.7 cm in diameter and 4.5 cm in height. Reflected power is reproducibly low, limiting the need for pretuning the microwave output to the sample. Microwave exposure can be controlled in 1 msecond increments with a range of 10 mseconds to 10 seconds. Mammalian cells and tissues fixed by this microwave device were evaluated by light and electron microscopy. Preliminary findings show large regions of excellent preservation in tissues and in cell suspensions in -100 mseconds.