Following the administration of L-selenomethionine-/sup 75/Se for a pancreas scan of a lactating mother, it was shown that there was a rapid and high level of incorporation of /sup 75/Se into the breast milk and the milk proteins. Between 5 and 29 hours after injection, the /sup 75/Se concentration in the whole milk and the milk solids decreased by a factor of more than 2; from 29 to 123 hours the concentrations appeared to decrease exponentially with a biological half-time of 78 +- 6 hours. At 123 hours post injection the concentrations of /sup 75/Se in the milk was 68% of that in the maternal plasma. An estimate of the /sup 75/Se intake by the infant was made from the measurements made on the single urine sample obtained 41 hours after the last breast feed. A whole body radiation dose to the infant was estimated to be about 500 ..mu..Gy, i.e. about one-tenth of the ICRP dose equivalent limit for a member of the general public. If breast feeding had been allowed to continue, the whole boent localization tumours and the principles of transferring the experimental data from animals to human beings.