The correlates of long-term protection from measles infection are poorly understood. We followed the development of measles-specific antibody and lymphoproliferative (LP) responses in 60 children for 6 months after MMR vaccination. Prevaccine plaque reduction neutralization antibody (PRN Ab) values were low (mean+/-SEM 9.9+/-1. 1). Ninety-three percent (56/60) had excellent PRN values at 6 months (PRN 1816+/-207). Prevaccine LP activity was also low (stimulation index (SI)=1.4+/-0.1) but increased rapidly (SI 10. 7+/-4.5 at 2-3 weeks; p<0.05). However, only 61% (37/60) of the children had both significant cellular and antibody responses (SI>/=3 and PRN>/=120: Ab(hi)CMI(hi)). One child had a strong LP response (SI=6.7) despite little antibody production (PRN=19 at 6 months: Ab(lo)CMI(hi)). We also conducted a cross-sectional study in a separate group of 87 children 5-13 years after MMR vaccination. PRN values >/=120 were present in most children at 5-8 (n=28) and 9-13 years (n=59) after vaccination (PRN 550+/-120 and 360+/-60, respectively) but a significant minority had either undetected or 'subprotective' values (29 and 34%, respectively). LP responses (SI>/=3) were detectable in 19/28 (66%) and 36/59 (56%) of the children 5-8 and 9-13 years after vaccination (SI 11.4+/-2.4 and 7. 75+/-1.9, respectively). Almost two thirds (18/28) of the children in the cross-sectional study with low or absent antibody titers (PRN 41+/-6) had strong LP responses to measles antigens (SI 6.8+/-1.3). These data suggest that LP responses may be better sustained than antibody titers in some children. The susceptibility of Ab(lo)CMI(hi) children to infection and the value of the early LP response for predicting the durability of immunity remain to be determined.