The objectives of this study were to evaluate lauric acid (LA) as a practical ruminal protozoa-suppressing agent and assess effects of protozoal suppression on fermentation patterns and milk production in dairy cows. In a pilot study, 6 lactating Holstein cows fitted with ruminal cannulae were used in a randomized complete-block design trial. Cows were fed a basal total mixed ration (TMR) containing (DM basis) 15% alfalfa silage, 40% corn silage, 30% rolled high moisture shelled corn, and 14% solvent soybean meal, and assigned to 1 of 3 treatments: 1) control, 2) 160 g/d of LA, or 3) 222 g/d of sodium laurate, which is equimolar to 160 g/d of LA, all given as a single dose into the rumen via cannulae before feeding. Both agents showed high antiprotozoal activity when pulse dosed at these amounts via ruminal cannulae, reducing protozoa by 90% (P<0.01) within 2 d of treatment. Lauric acid reduced ruminal ammonia concentration by 60% (P<0.01) without altering DMI. Both agents reduced ruminal total free AA concentration (P<0.01) and LA did not affect ruminal pH or total VFA concentration. In a large follow-up feeding trial, 52 Holstein cows (8 with ruminal cannulae) were used in a randomized complete-block design trial. Cows were assigned to 1 of 4 diets and fed only that diet throughout the study. The TMR contained (DM basis) 29% alfalfa silage, 36% corn silage, 14% rolled high moisture shelled corn, and 8% solvent soybean meal. The 4 experimental diets were similar, except part of the finely ground dry corn was replaced with LA in stepwise increments from 0 to 0.97% of dietary DM, which provided (as consumed) 0, 83, 164, and 243 g/d of LA. Adding these amounts of LA to the TMR did not affect DMI, ruminal pH, or other ruminal traits, and milk production. However, LA consumed at 164 and 243 g/d in the TMR reduced the protozoal population by only 25% and 30% (P=0.05), respectively, showing that these levels, when added to the TMR, were not sufficient to achieve a concentration within the rumen that promoted the antiprotozoal effect of LA.