A recently submitted question:
How to get Fresh blood out of Bed Sheets. The key is to keep the stain wet, rinse out under the tap in cold water. If the stain is on the mattress sponge out with water (check out the tip in the Howtocleananything.com database on getting blood out of carpet the solution is the same for a mattress).
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If you have successfully rinsed the stain just launder as usual in cold water. If the stain is old soak in cool salt water for a long time (hours) and rinse thoroughly. Soak again in water with a some ammonia. Launder as above. Since blood is organic you can try soaking in an enzyme solution for a half an hour and launder. As a last resort bleach if the bed sheets are white. Heat will set this stain.
Always test a small area first before proceeding with any stain removal technique.