Introduction - To best determine the optimal age for vaccinating infants against measles, it is useful to understand the dynamics of maternal anti-measles neutralizing antibodies, which can neutralize the vaccine before an immune response is induced. This article presents the dynamics of these antibodies in infants in France. Materials and Methods - Serum levels of maternal anti-measles antibodies in 348 infants aged 0 to 15 months were measured using the plaque reduction neutralization assay. Results - The geometric mean of measles-neutralizing antibody titers decreased rapidly with age, from 1,740 mIU/mL [95% CI: 1,218–2,487] in the 0–1-month age group to 223 mIU/mL [95% CI: 120–413] (5–6 months) and 65 mIU/mL [95% CI: 40–107] (6–7 months), with a significant difference (p<0.001) between the 3-month age groups. The proportion of infants with protective antibody levels (≥ 120 mIU/mL) dropped from 100% [95% CI: 83.2–100] among infants aged 0–1 months to 10% [95% CI: 2.1–26.5] at 6–7 months of age. Conclusion - After 6 months, fewer than 10% of infants are protected against measles by maternal antibodies. Herd immunity, driven by increased vaccination coverage, could help protect infants during the vulnerable period between passive and active protection. (R.A.)