What to Look for in Pet Store Mice:
A long, sleek, slim (not skinny) body, with a smooth, straight tail, bright eyes, open, clear ears, long, healthy vibrissae (whiskers), and symmetry of the head and body.
The ability to stretch is important. The mice that you choose should be willing and able to stretch up onto their toes to reach a ledge, the wheel, etc. If they appear hunched and do not stretch even when they are walking around, avoid those mice as they are most likely sickly.
Do the mice appear to be happy, energetic, friendly, and charismatic? They should not bite or leap out of your hand when you are holding them. Keep in mind that it is normal for pet store mice to run away from your hand if they do not know you. However, they should NOT bite you if you pick them up by the base of the tail, or if you corner them and scoop them up. Avoid biters.
Inspect the ears inside and out. If the mouse has ripped, bitten, crusty, or very red ears, avoid that mouse. Rips and bites area telltale sign that that mouse has been involved in a fight. This could mean that the mouse in question is very aggressive.
NO Sniffling, Sneezing, Coughing, Raspy Breathing, or Laboured Breathing: Hold the mouse to your ear. If you hear a wet sneezy sound, a cough, rasp, wheeze, etc., you should avoid that mouse. If the mouse is making this sound, avoid ALL of the mice in that tank. Mycoplasmosis is common in pet store mice. Even though the individual you choose is not showing any symptoms at that time, it may develop symptoms later on. Mycoplasmosis is very contagious from mouse to mouse, hence the reason to avoid all other mice in that tank. There are always risks with purchasing mice from pet stores instead of breeders. Muffled vibrations are very common in healthy mice. If you hear these, do not be alarmed.
Look at the conditions in which the mice are being kept. This will give you a good idea as to the quality of the animals. The cleanliness of the cage can tell a lot as to how the mice are kept, and what condition they are currently in. Bedding is probably the most important factor in choosing a mouse from a pet store. If the mice are being kept in pine or cedar bedding, regardless of what the pet store workers tell you, the mice are at risk for liver disease, respiratory failure (including mycoplasmosis), skin irritation, etc. Do not choose mice from pet store that uses cedar or non-kiln dried pine.
Look under the fur by gently brushing it backward with your fingertips. Look carefully at the skin below. If you see any scabs, crust, or open wounds, do not choose that animal