Dog blood levels are critically low with the retirement of Melbourne University's regular donor.

The university vet hospital's largest canine blood donor, a 56kg Leonberger called Daisy, has saved the lives of three dogs with every blood donation, but her retirement has led to a call for more donors.

Dogs are able to donate blood four times per year, helping up to 12 other dogs. Doggie blood donors must be over 25kg and aged one to seven, with a calm temperament.