Bulk tank milk samples collected from 175 dairy herds throughout Wisconsin averaged 466 micrograms of iodine per liter with 11% of the samples containing greater than 1000 micrograms per liter. From questionnaires on feeding and management, use of iodine supplements in feed was associated with increased iodine in milk. One gram of ethylenediamine dihydriodide was fed daily for 2 wk to dairy cows with a history of mastitis. Milk iodine increased from 210 to 6225 micrograms per liter. There was no significant effect on somatic cells, mastitis organisms, or incidence of clinical mastitis.