A reproducible test is described for the measurement of serum neutralizing capacity against a strain of CDV adapted to the chick embryo. A heat-stable neutralizing substance against CDV was demonstrated in human serum and in high titer in the gamma globulin fraction. The neutralizing substance is transmitted via the placenta quantitatively from the mother to the newborn infant. It is lost by the sixth month and reappears in the population between the second and tenth years of life. It reaches an incidence of almost 100 per cent in adults. The evidence so far available indicates that the neutralizing substance has the properties of a true antibody, formed in response to an as yet unidentified infection with an agent identical to or bearing an antigenic structure in common with CDV.