The iodine concentration in the chain sow feed-->blood serum of dams-->milk-->blood serum of piglets (2 per litter) was determined in 36 litters in two experiments with different dietary iodine levels and in 16 litters in the framework of field studies in two piglet production farms in each case at weaning after four weeks lactation. In the blood serum also the concentration of thyroxine (T4) and triiodothyronine was determined. The serum concentration of iodine and T4 did not indicate dietary iodine administration (sows) or showed only a weak response (piglets). The iodine concentration of milk was very strongly affected by iodine administration and by the iodine status of sows prior to the experiments. The highest milk iodine concentration was found in the sows of piglet production farms, corresponding to the level of iodine administered. For diagnosis of the iodine supply status the iodine concentration of sow milk should be analyzed. The lower limit of milk iodine concentration is presently defined as 50 micrograms/l, and the mean of 5 random samples per sow herd should not fall below this limit. Serum concentrations of iodine and T4 may remain moderate even in case of a low iodine supply (sow serum: 30 micrograms iodine/l, 25 nmol T4/l; piglet's serum: 50 micrograms iodine/l, 55 nmol T4/l), and are unsuited for diagnosis of iodine status.