Establishing Mouse Breeding Groups




This policy outlines guidelines for establishing and maintaining mouse breeding groups in accordance with the 2011 version of the Guide for the Care and Use of Laboratory Animals. The guide recommends 51 square inches for a female mouse with a litter but suggests using performance-based criteria to establish institutional guidelines for housing density.

The UIUC Institutional Animal Care and Use Committee (IACUC) has determined that mouse lines may be maintained in continuous trio mating under the following circumstances:

  • Strains have an average litter size of eight or fewer pups.
  • Litters are weaned at 21 days of age without exception.
  • Trios are established to minimize the age differential between litters with the cage (i.e., set up together on the same date).
  • Trio mating has been demonstrated to be successful for this particular line.


Historically, mice on the University of Illinois campus have been bred in monogamous pairs or trios (one male/two females) in cages with approximately 80 square inches of floor space. Breeding performance indices support the practice of trio mating for certain strains of mice. Both the NIH Office of Laboratory Animal Welfare and the Association for Assessment and Accreditation of Laboratory Animal Care has stated that standard mouse cages may be appropriate for trio breeding. The European convention (ETS 123) also states that 51 square inches are acceptable for trio breeding schemes.


The principal investigator is responsible for ensuring that the breeding colony management complies with these guidelines. If there are decreases in breeding performance indices (decreased pup size, decreasing numbers of pups surviving to weaning) or noncompliance with weaning practices, mating systems may be re-evaluated in consultation with the Division of Animal Resources.

Approved Date

Revised Date