Three different localisations of each one of 118 udder quarters were histologically tested. All the 118 quarters had been clinically unchanged for not less than three months before slaughter, and the milk obtained from them had been without any major change over at least six weeks. Sixty-two quarters that had produced positive response to the highspeed mastitis test just before slaughter exhibited 45 inflammatory changes (72 per cent), most of them in terms of proliferative lobular mastitis or exudative alveolitis. One of such changes was recorded from 20 in 47 quarters (21 per cent) which had reacted negatively to the highspeed mastitis test. Comparison with the clinical history of the udders showed that 47 per cent of the established changes could be interpreted as sequels of previous clinical disorder in udder condition, while the greater part of all findings was based on completely negative udder histories.