Glutaraldehyde stabilized red blood cells are produced from freshly washed dog RBCs by brief exposure to a glutaraldehyde-buffer solution and exhaustive washing in saline. This treatment largely preserves the native antigenicity of the erythrocytes while rendering the cells generally resistant to lysis by ostmotic shock, freeze-thawing or immune hemolysis. This reagent can be used directly in hemagglutination procedures, or may be coupled with various proteins for hemagglutination testing.