Housing Guidelines for Mice

Reference: The GUIDE for The Care and Use of Laboratory Animals, Institute of Laboratory Animal Resources, NRC 1996

The Guide specifies housing requirements for most animal species and is the only reference for housing of laboratory rodents not regulated by the Animal Welfare Act (AWA). The most recent publication restricts the housing of mice to the following:


Up to 15 grams 8 sq inches per animal
Up to 25 grams 12 sq inches per animal
> 25 grams >15 square inches per animal

Caging density based upon above housing guidelines and available caging at MSSM

Icahn Medical Institute
Allentown JAG cage = 75 sq inches of floor space
Up to 25 grams @ 5 mice x 12 sq inches/mouse = 5 adult mice per cage
>25 grams - 5 adult mice per cage

Annenberg, Atran Building
Two caging systems available, two primary.

  1. Regular Allentown cage = 67.0 sq inches of floor space.
    Up to 25 grams - 5 adult mice per cage
    >25 grams - 4 adult mice per cage
  2. Allentown JAG cage = 75 sq inches in floor space
    5 mice x 12 sq inches/mouse = 5 adult mice per cage

General guidelines - It should be assumed that most adult mice weigh 25 grams or greater and the above guidelines should be applied. The guidelines however do not adequately address the housing density for cages with breeding animals. The following densities are recommended for breeding colonies in all buildings at the MSSM:

One female and litter up to 21 days of age
Three females with one male
One male and 1-2 females
Two females and male with one litter up to 21 days of age

Two litters in one cage (two generations or two separate litters) other than a breeder cage will be considered overcrowding (see CCMS Policy on Overcrowding).


Recently approved IACUC guidelines establish the housing densities for mice, particularly mice involved in breeding protocols. It should be remembered that the CCMS husbandry procedure is to change mouse cages once weekly. This change out rate is based upon the approved stocking densities. Overcrowding of mice results in humane concerns as these animals are subjected to increased ammonia levels (urination), fecal contamination, and psychological distress. When animals are found to be in overcrowded conditions the CCMS husbandry staff will identify the cage with an OVERCROWDED card. Investigators will have 24-72 hours (depending on the severity of overcrowding) to separate the flagged cages. Notification will be via e-mail only. The "clock" begins running when the email notice is sent. If overcrowding is severe the supervisor or veterinary staff will reserve the right to separate the animals without notification for humane concerns. Overcrowded cards must not be removed from cages until cages have been separated. Additional charges will be accessed if the husbandry staff reports that cards are removed by investigators prior to separating reported cages. If investigators disagree with the need to separate animals they should consult the IACUC for a variance. All litters over 21 days of age are considered overcrowded unless prior IACUC approval has been granted. Specific times of extension must be given when IACUC extension is approved and noted on a Special Services Form and posted within the room, i.e. weaning at 28 days.

Summary Overcrowding Policy

  1. Cage densities outside of approved housing guidelines will be flagged with OVERCROWDED card.
  2. Notification will be via e-mail
  3. Separation of reported cages must be done by 72 hours.
  4. Cages not separated by 3 days will be separated by CCMS at charge of $50.00 per reported cage.
  5. Cards removed without separating cages. Cages will be separated by CCMS husbandry staff and charged $75.00 per new cage generated.
  6. Investigator must have prior IACUC approval to deviate from approved IACUC housing standards.
  7. Investigators may submit a Special Services Request for CCMS staff to AUTOMATICALLY separate overcrowded cages without prior notification. CCMS will separate these cages at NO CHARGE. A Special Services Request must be on file for this service to apply.