Red blood cells interact with glutaraldehyde (GA) in a complex kinetic pattern of events. At a given GA concentration in phosphate buffered saline (PBS), the sequence of cell 'volume' response, as measured by resistive pulse spectroscopy (RPS), includes: an immediate response to the overall solution osmolality; a constant volume, latent phase; a rapid swelling phase; an intermediate constant volume phase; and a shrinkage phase to a final steady state volume. The final volume depends on fixative solution osmolality; for GA concentrations between 0.05% and 0.25% w/v, fixative osmolalities of less than 355 mosM, including 'isotonic', or greater than 355 mosM, lead to final cell volumes greater or less than native, respectively. Cell-membrane deformability decreases continuously and monotonically with time, as assessed by RPS. The rate of fixation is a direct function of GA concentration, in accordance with a derived empirical expression. The measured kinetic responses are related to considerations of cell size, deformability, and form, and to mechanisms involved in abrupt osmotic hemolysis.