To study the kinetics of glutaraldehyde fixation, fresh rabbit liver cubes were immersed in 3% buffered 3H-glutaraldehyde for various periods of time. Following weighing and a brief rinse in water, the tissues were solubilized, and the radioactivity was measured in a scintillation counter. Binding of the isotope was half-maximal after approximately 4 h and a plateau was reached after approximately 20 h. We also investigated the reversibility of glutaraldehyde fixation. Fixed liver cubes were weighed and immersed in water for various periods of time, and after solubilization, the radioactivity was determined. After rinsing for 48 h, approximately 95% of the radioactivity was lost from the tissue specimens, indicating that fixation with glutaraldehyde is largely reversible. Light and electron microscopy of specimens rinsed for 1 and 48 h showed essentially similar morphology. Rinsing for 48 h restored some of the immunoreactivity that was absent after rinsing for only 1 h.