Eighty-five children received three injections of a vaccine consisting of Haemophilus influenzae type b (Hib) capsular polysaccharide (CPS) conjugated to tetanus toxoid (TT) (Hib-TT) at 3, 5 and 12 months of age according to the vaccination schedule for Swedish children. Diphtheria-tetanus toxoid vaccine was concurrently injected at another site. Two dosages, 7.5 and 15 micrograms, of Hib CPS were studied. No serious reactions occurred. Hib-TT elicited fewer local reactions than diphtheria-tetanus toxoid vaccine. Significant increases in Hib CPS serum antibodies occurred after all injections in both dosage groups with virtually no differences between the two groups. After the first and second injections geometric mean serum antibody concentrations of both dosage groups combined increased to 0.49 and 3.71 micrograms/ml and 81 and 99% of the vaccinees, respectively, had concentrations greater than 0.15 micrograms/ml. After the third dose geometric mean concentrations increased to 13.7 micrograms/ml and all had concentrations greater than 0.15 micrograms/ml. The geometric mean Hib CPS antibody concentrations decreased to 1.24 micrograms/ml 18 months after the third injection, but 97% still had concentrations greater than 0.15 micrograms/ml. The rise of Hib CPS antibodies was mostly in the IgG class. The most pronounced increase was seen in the IgG1 subclass but there were also increase in IgG2 and IgG3. Protective concentrations of TT antibodies were found in all postimmunization sera. In conclusion Hib-TT is safe and immunogenic in infants and should be protective from 6 to 30 months and probably longer thereafter.