Dietary conjugated linoleic acid (c9,t11/t10,c12-CLA), but not various dietary trans-18:1 isomers, stimulates mammary gland growth in ovariectomized mice. Representative images of whole mounts from inguinal mammary glands of 42 days old mice fed either the (a) control diet (TFA-CON) or that with 5% fat as (b) partially hydrogenated linseed oil (PH-LIN), (c) partially hydrogenated safflower oil (PH-SAF), (d) partially hydrogenated sunflower oil (PH-SUN), or (e) 2.7% conjugated linoleic acids (c9,t11/t10,c12 CLA). Mice were ovariectomized at 21 days, then fed the experimental diets from 22 days until euthanasia at 42 days of age. Scale bar 2 mm. f Mammary ductal elongation was the distance from the teat to the furthest-reaching ductal terminus. Data are mean ± SEM (n = 6–8/diet). a, b, cMeans with different letters are different (P < 0.05)