The trans fatty acids as well as cis-9 trans-11 C18:2 (CLA) were determined in the colostrum and milk fat of individual cows during up to 82 days of lactation. The analyses were performed using gas chromatography (GLC) on a capillary column (CP Sil 88) combined with the method of argentation thin-layer chromatography (Ag-TLC). The results obtained in the study indicated that the milk fat of individual cows, kept under identical living and feeding conditions, was characterised by an extremely high variability of trans isomers concentration, especially of those of C18:1 acid. The differences referred not only to the total content of trans C18:1 isomers but also to the proportion of trans-10 and trans-11 isomers of C18:1. In some fat samples, a high concentration of trans C18:1 isomers was observed and trans-10
isomer of C18:1 was found to dominate. In the case of all the cows, the colostrum fat was characterised by a lower average content of the examined acids as compared to the milk fat. The average value of CLA in the colostrum fat of all the cows under study was 0.34%, and in the milk fat 0.42% of total fatty acids.