The safety and immunogenicity of a vaccine against Haemophilus influenzae type b consisting of purified polyribosylribitolphosphate conjugated to tetanus toxoid (PRP-T) was evaluated in 278 Chilean infants who were randomly assigned to one of three treatment groups: Group A, PRP-T mixed with diphtheria-tetanus toxoids-pertussis (DTP) vaccine in a single syringe and given as a single inoculation in one arm and placebo in the other arm; Group B, PRP-T given in one arm and DTP in the other arm; Group C, DTP given in one arm and placebo in the other. Infants were immunized at 2, 4 and 6 months of age and examined daily for 4 days after each immunization; serum PRP antibodies were measured at baseline and 2 months after each dose. The only adverse systemic reaction attributable to PRP-T beyond that caused by DTP alone was a 7 to 20% increase in febrile responses in the first 24 hours after the first and second doses of vaccine; the fevers were largely low grade and not accompanied by increased irritability, diminished activity or loss of appetite, compared with the group who received DTP without PRP-T. After the first dose 72% of infants who received PRP-T combined with DTP and 67% who received it in a separate arm attained antibody concentrations greater than or equal to 0.15 micrograms/ml. After two doses of PRP-T, 93 and 95%, respectively, had concentrations greater than or equal to 0.15 microgram/ml and after three doses 100% of infants who received PRP-T had such titers.(ABSTRACT TRUNCATED AT 250 WORDS)