A competitive ELISA method is described for the measurement of total antibodies to the capsular polysaccharide of Haemophilus influenzae type b (HibCPS) in human sera. The competitive method showed an excellent correlation to the radioantigen binding assay (RABA, or Farr assay) and improved correlation of sera with low titers with respect to the more conventional noncompetitive method. Overestimation of samples in the low concentration range was no longer observed with the competitive ELISA method. The free HibCPS competition allowed us to eliminate the day-to-day background variation typical of some sera; thus, only values representing the true anti-HibCPS response were determined. The use of precoated microplates, which could be stored up to 8 months, greatly improved the speed of the procedure. An overall correlation coefficient of 0. 9660 was found when 407 serum samples with a wide variety of anti-HibCPS antibody levels were tested with the competitive ELISA and RABA. The regression line was very close to the ideal line, with a slope of 1.0045 and an intercept of -0.1996. A subset of 96 serum samples representative of all pre- and postimmunization samples was used to compare the competitive ELISA with a previously described ELISA method. The competitive method performed in two laboratories in different countries showed a better correlation with the RABA. The correlation factors were 0.9770 and 0.9816, respectively, while a factor of 0.9547 was found with the previously described noncompetitive procedure, which was better for this method than previously reported (r = 0.917). Therefore, the competitive ELISA is proposed for the assay of anti-HibCPS titers in sera from vaccinated subjects.