Groups of 10-day-old calves and 5-month-old lambs were bruised 48 hours, 24 hours, 8 hours, or the instant before slaughter. By gross examination, 48-hour-old calf bruises had a characteristic red-yellow colouration, but other bruises could not be differentiated reliably. By microscopic examination, bruises of all ages could be differentiated by cellular criteria common to both species. Unlike fresh lesions, 8 hour bruises contained numerous neutrophils. Twenty-four hour old bruises contained equal numbers of macrophages and neutrophils and at 48 hours macrophages, fusiform cells and new capillaries were present in the damaged muscle. The criteria established in this study could be used to determine the ages of spontaneous bruises in abattoir carcases and so provide a basis for an investigation aimed at detection of the traumatic episode from which they result.