Detailed data from a colony of C57BL mice kept for research into ageing processes are given on weights of 13 organs between the ages of 6 and 30 months; pathological findings and x-ray results from animals aged 6, 24 and 30 months; age-specific tumour incidences in 24 organs in 3-month period up to 36 months of age. Results of previously published age-related studies of reproductive capacity, dental abnormalities and brain changes are summarised.