1. The diphtheria antitoxin titres of several thousand samples of cord blood have been analysed with relation to the histories of the mothers.
2. Among mothers without histories of diphtheria or of immunization those born in 1933 or later showed a higher degree of natural immunity.
3. The distribution of antitoxin titres among mothers who had suffered from diphtheria was similar to the average distribution among the unimmunized.
4. The differences in proportions with any antitoxin in different groups of unimmunized mothers were almost entirely made up of those with titres above 0·04 and below 1·0 unit per ml.
5. Among the artificially immunized mothers there were many more with low titres, i.e. below 0·04, and a few more with high titres, i.e. over 1·0, than among the unimmunized.
6. Mothers immunized between the ages of 9 and 14 with the peak at 11 or 12 years showed the highest immunity rate.
7. The greatest number of high titres occurred among recovered cases subsequently immunized.