Reduction of the number of injections necessary to confer protection in the infant schedule would reduce discomfort, improve cost-effectiveness and create space for the addition of new vaccinations in the future. This study assessed the immunogenicity of one, two or three doses of meningococcal C conjugate vaccine conjugated to tetanus toxoid (MCC-TT) [Neis-VacC] given concomitantly with a combined diphtheria/tetanus/acellular pertussis/Haemophilus influenzae type b -TT conjugate (DTaP-Hib-TT) [Infanrix-Hib] vaccine at 2, 3 and 4 months of age. A total of 106 healthy UK infants were enrolled and randomised into two groups, one in which blood was taken after the first and third dose and the other after the second and third dose. The meningococcal serogroup C serum bactericidal antibody (SBA) geometric mean titre (GMT) rose significantly from post-first dose (491, 95% CI 275, 877) to post-second dose (1052, 95% CI 774, 1433) (p=0.03), with no significant change after the third dose (1024, 95% CI 768, 1366). An SBA titre of >or=8 was achieved by 92% after the first dose and 100% after the second and third doses. The Hib IgG geometric mean concentration (GMC) rose significantly after each dose: post-first (0.14 microg/ml 95% CI 0.10, 0.18), post-second (0.54 microg/ml, 95% CI 0.33, 0.90), post-third (2.04 microg/ml, 95% CI 1.52, 2.74). The Hib GMC after the third dose was higher than reported previously when this DTaP/Hib was given either on its own or concomitantly with a MCC-CRM conjugate vaccine according to the UK 2, 3 and 4 month schedule. This suggests some enhancement of the response to a Hib-TT vaccine by concomitant administration of MCC-TT. These results suggest that a reduced number of doses of MCC-TT would be adequate in infancy if given concomitantly with an acellular pertussis-containing vaccine.