We evaluated the effects of abomasal infusion of an emulsifier (Polysorbate 80, Tween 80) on fatty acid (FA) digestibility and production responses of lactating dairy cows. Eight rumen-cannulated cows (109 ± 18 DIM) were randomly assigned to a treatment sequence in replicated 4 × 4 Latin squares with 18-d periods including 7 d of washout, and 11 d of infusion with sampling on the last 4 d. Treatments were abomasal infusions of water carrier only (CON) and 3 levels of increasing doses of Tween 80 delivering 15 (D-15), 30 (D-30), and 45 (D-45) g/d. The Tween 80 was dissolved in water before infusions, which were delivered at 6-h intervals. Cows were fed the same diet which contained (% DM) 31% NDF, 17% CP, 25% starch and 4% FA (2% DM from a saturated FA supplement containing 33% C16:0 and 51% C18:0). The statistical model included the random effect of cow within square, and the fixed effect of treatment, period, square, and their interactions. Results in the text are presented in the following sequence: CON, D-15, D-30 and D-45. Increasing Tween 80 infusion quadratically increased apparent total-tract digestibility of total FA (60.7, 65.3, 70.9, and 66.8%, P = 0.05), 16-carbon FA (61.7, 63.9, 70.4, and 66.7%, P = 0.04), and 18-carbon FA (59.8, 65.6, 71.1, and 66.6%, P = 0.04). Increasing Tween 80 quadratically increased absorbed total FA (625, 670, 744, 658 g/d, P = 0.01), 16-carbon FA (151, 157, 197, and 157 g/d, quadratic, P = 0.04), and 18-carbon FA (420, 460, 500, 444 g/d, P = 0.01). The D-45 treatment tended to reduce DMI compared with the other treatments (29.0, 28.8, 29.6, and 27.6 kg/d, quadratic, P = 0.09). Increasing Tween 80 infusion quadratically increased milk fat content (3.23, 3.35, 3.45, and 3.35%, P = 0.03), milk fat yield (1.54, 1.61, 1.65, and 1.55 kg/d, P = 0.02), ECM (45.7, 46.9, 47.5, and 45.3 kg/d, P = 0.03), and plasma NEFA concentration (95.6, 98.4, 101.2, and 98.6 μEq/L, P = 0.05). There was no effect of treatments on milk yield (47.9, 48.3, 48.0, and 46.6 kg/d, P = 0.12). In conclusion, infusing an exogenous emulsifier improved FA digestibility and milk fat yield responses when cows were fed a diet containing a saturated FA supplement.