The concentration of tri-iodothyronine (T3) and thyroxine (T4) in human milk was determined by radioimmunoassay (RIA). The analysis of T3 was performed on unextracted milk and on ethanol extracts of defatted milk. Analysis of unextracted milk was complicated by artifacts. Reliable and reproducible results were achieved only with the milk extracts. In 10 colostral milk samples the mean T3 levels +/- SD were 0.80 +/- 0.52 nmol/l before feeding (early milk) and 0.93 +/- 0.62 after feeding (hind milk). The T3 concentration in colostrum did not change significantly during the feeding to the infant. In 12 mature milk samples collected between infant feedings, the mean T3 +/- SD was 1.19 +/- 0.42 nmol/l. T4 was not detected in any of the samples analysed (detection limit 3 nmol/l).