Stimulated by questions raised on potential differences between the Haemophilus influenzae type b capsular polysaccharide (PRP) vaccines on the market and the applicability of the efficacy data of a similar PRP vaccine obtained in a large field study in 1974 in Finland, we wished to compare the immunogenicity of all these preparations in 24-month-old Finnish children. In our study 137 children received the now recommended 25-micrograms dose of 1 of 4 H. influenzae type b PRP vaccines currently available, and an additional 86 children received half this dose corresponding to the 12.7 micrograms used in 1974. Anti-PRP antibodies were measured in blood samples taken before and 4 weeks after vaccination by the same radioimmunoassay and in the same laboratory as in 1974. The vaccines were equally immunogenic. Furthermore the now recommended dose of 25 micrograms did not give results (geometric mean concentrations, 2.38 to 3.45 micrograms/ml) differing from those after a 12.5-micrograms (2.01- to 3.45-micrograms/ml) dose which was used in 1974. Antibody concentrations of 0.15 and 1.0 micrograms/ml were achieved in 91 to 95 and 66 to 84% of the children, respectively. The corresponding values after 1974 vaccinations were 3.53 micrograms/ml and 100 and 82% of children, respectively. The percentage of those responding with concentrations greater than or equal to 1 microgram/ml was somewhat higher in these Finnish children than reported for children of the same age receiving the same vaccine lots in the United States. Adverse reactions were mild or moderate and transient.