A nationally marketed product for dipping cows' teats after each milking was tested for efficacy in preventing new intramammary infection. Teats on one side of the udder of 150 Jersey cows were dipped routinely for 3 months in the elemental iodine-in-oil product at 0.5% iodine concentration and then for 3 months in the product at 1.0% iodine. The opposite teats served as undipped controls. Intramammary infection was determined by evaluation of monthly quarter foremilk samples. At the higher iodine concentration, use of the teat dip had no effect on incidence of new infection; at the lower concentration, it was associated with an increase in new infection by Staphylococcus aureus (P<0.05).