To test for immunologic memory aftera single dose of meningococcal C conjugate (MCC) vaccine in toddlers, 226 children 12-18 months old were randomized to receive 1 of 3 MCC vaccines, with a C polysaccharide booster 6 months later. The protein conjugate was diphtheria mutant toxoid in 2 vaccines (MCC-CRM197) and was tetanus toxoid in the third (MCC-TT). One month after the MCC vaccines, 91%-100% of children had serum bactericidal antibody (SBA) titers ≥8, and 89%-100% had a ≥4-fold increase. Geometric mean titer (GMT) increased from <4 to 215 (95% confidence interval [CI], 166-279). MCC-TT induced higher SBA GMTs (P < .001) and higher proportions with SBA ≥8 (P = .02) than did the MCC-CRM197vaccines. By 6 months, GMTs had decreased to 55.1 (95% CI, 40-76), but IgG antibody avidity increased (P < .001). Induction of immunologic memory was confirmed by a GMT of 1977 (range, 1535-2547) after the polysaccharide booster and a further increase in avidity. This evidence justified the use of a single dose in a catch-up immunization program for children 1-18 years old.