By challenge experiments on minks it has been shown that distemper vaccines may provoke formation of neutralizing antibodies in good titres without giving protection. In small scale experiments it was found that mink kits even at the age of 4 weeks might be protected through vaccination, provided the mothers had not been vaccinated. In kits born of vaccinated mothers, successful vaccinations may apparently be carried out by the age of 11 weeks, but if applied before this time, maternal antibodies seem to interfere with the vaccination. At 4 weeks, maternal antibodies alone may give protection.