The concentrations of CLA isomers were determined by Ag+ -HPLC in the milk fat of cows fed a control diet consisting of hay ad libitum and 15 kg of fodder beets or this diet supplemented with oilseeds containing either high levels of oleic acid (rapeseed), linoleic acid (sunflower seed), or alphalinolenic acid (linseed). Highly significant (P < or = 0.001) correlations were found between the daily intakes of oleic acid and the concentration of the CLA isomer trans-7,cis-9 in milk fat, of linoleic acid and the CLA isomers trans-10,trans-12, trans-9,trans-11, trans-8,trans-10, trans-7,trans-9, trans-10,cis-12, cis-9,trans-11, trans-8,cis-10, and trans-7,cis-9, and of alpha-linolenic acid and the CLA isomers trans-12,trans-14, trans-11 ,trans-13, cis,trans/trans,cis-12,14, trans-11 ,cis-13, and cis-11 ,trans-13. CLA concentrations were also determined in the milk fat of cows grazing in the lowlands (600-650 m), the mountains (900-1210 m), and the highlands (1275-2120 m). The concentrations of many isomers were highest in milk fat from the highlands, but only three CLA isomers (cis-9,trans-11, trans-11 ,cis-13, and trans-8,cis-10) showed a nearly linear increase with elevation. Therefore, these three CLA isomers, and particularly the CLA isomer trans- 11,cis-13, the second-most important CLA in milk fat from cows grazing at the three altitudes, could be useful indicators of milk products of Alpine origin.