The welfare of conventional stock laboratory mice has been assessed in 46 UK animal units using an expert-defined welfare assessment protocol containing 119 measures of mouse welfare. These were recorded using a questionnaire and observations made during a one-day visit to each unit. The standard of mouse welfare was considered to be good with widespread use of substrate and nesting material and space allowances in most cases well above the minimum recommended levels. Education and training was available and encouraged by the majority of animal units. The health and welfare of laboratory mice was being frequently assessed by animal care staff using daily inspections/observations, health records, and health monitoring schemes. Overall the mice assessed could be considered to be in good health, as indicators of poor health and welfare were exhibited at low levels, and the mice were observed exhibiting a wide range of positive natural behaviours. A number of environmental conditions (humidity, noise and light intensity) were outside recommended ranges in some animal units. The provision of cage resources such as shelters, gnawing material, floor food and other enrichment items were found to be variable. A high proportion of the units surveyed housed at least some of their mice (mainly males) singly and handling of mice by care staff varied between units. In some units there may be an opportunity for some staff to improve in some aspects of mouse handling. Finally, a number of interesting correlations were found between various behaviours and potential indictors of abnormal health or welfare, which require further investigation.