Healthy adults, 18-55 years old, were immunized once with a tetravalent (serogroups A, C, Y, and W-135) meningococcal vaccine conjugated to diphtheria toxoid at 1 of 3 doses and were monitored for safety, reactogenicity, and immunogenicity. No immediate reactions were observed. Only 1 of 89 subjects reported fever; only 1 reported any severe reactogenicity (local pain/soreness, chills, arthralgia, anorexia, and malaise). For each serogroup and in each dose group, the geometric mean serum bactericidal antibody (SBA) titer and immunoglobulin G concentration increased after immunization. In the 4- and 10-microg-dose groups, all subjects had SBA titers >/=8 against serogroups A and C, and 89% and 93% of subjects had SBA titers >/=8 against serogroups Y and W-135, respectively. The A, C, Y, and W-135 Neisseria meningitidis-diphtheria toxoid conjugate vaccine, when given to healthy adults as a single intramuscular injection of 1, 4, or 10 microg/serogroup, is acceptably tolerated and immunogenic and deserves further development.