Leucocrystal Violet reacts with the heme-group in blood to give a violet color. Its application can be used to identify bloodstains that are not visible and to enhance and provide additional contrast to bloodstain patterns that are visible.  This reagent contains hydrogen peroxide, so if used on heavy bloodstains foaming may result and cause loss of detail.  Leucocrystal violet is typically used on porous surfaces.

Leucocrystal Violet reacts with heme, and therefore is a presumptive test for blood.  However, it is NOT the presumptive test for blood used by the Laboratory.  Presumptive testing for blood is done using phenolphthalein reagent.

The images below show two examples of enhancement of bloody impressions on carpet.  The top row of images show a footwear impression while the bottom two images show numerous impressions that turned out to be paw prints from a family dog.