Thirty lactating dairy cows were used in a 333 Latin-square design to investigate the effects of a raw or extruded blend of linseed and wheat bran (70:30) on plasma and milk fatty-acids (FA). Linseed diets, containing 16.6% linseed blend on a dry-matter basis, decreased milk yield and protein percentage. They decreased the proportions of FA with less than 18 carbons in plasma and milk and resulted in cis -9, cis -12, cis -15 18:3 proportions that were more than three and four times higher in plasma and milk, respectively, whereas cis -9, cis -12 18:2 proportions were decreased by 10–15%. The cis -9, trans -11, cis -15 18:3 isomer of conjugated linolenic acid was not detected in the milk of control cows, but was over 0.15% of total FA in the milk fat of linseed-supplemented cows. Similarly, linseed increased plasma and milk proportions of all biohydrogenation (BH) intermediates in plasma and milk, including the main isomer of conjugated linoleic acid cis -9, trans -11 18:2, except trans -4 18:1 and cis -11, trans -15 18:2 in plasma lipids. In milk fat, compared with raw linseed, extruded linseed further reduced 6:0–16:0 even-chain FA, did not significantly affect the proportions of 18:0, cis -9 18:1 and cis- 9, cis -12 18:2, tended to increase cis -9, cis -12, cis -15 18:3, and resulted in an additional increase in the proportions of most BH intermediates. It was concluded that linseed addition can improve the proportion of conjugated linoleic and linolenic acids, and that extrusion further increases the proportions of intermediates of ruminal BH in milk fat.