We used in-depth interviews with 23 mothers of babies aged 1-2 years to explore their response to the addition of Haemophilus influenzae type b (Hib) vaccine to the primary schedule. Acceptability of the vaccine was principally attributable to maternal perceptions as to the severity of meningitis, with a tendency to overestimate the efficacy of the vaccine. Advice from health professionals and the behaviour of peers contributed to its acceptance. Barriers to the uptake of the vaccine included suspicion regarding the newness of the vaccine, a fear of vaccine overload in such young babies, and the distress of two separate injections. There may be a limit to the number of antigens, and particularly the number of injections, acceptable to mothers.