To evaluate the comparability of immunoassays, the Center for Biologics Evaluation and Research organized an international collaborative study in which 33 laboratories participated. For a coded panel of 21 samples, each laboratory measured IgG antibodies to specific proteins of Bordetella pertussis using assay systems currently in place. Analyses were performed to evaluate the assay precision and the quantitative agreement among laboratories. Data from a subset of 12 laboratories are used to illustrate points relevant to the use of immunoassays in seven vaccine efficacy studies. Differences among the laboratories in assay precision for samples with known two-fold differences indicate that serological case definitions must take into consideration the characteristics of the assays and the concentration of antibody in the samples. Assays performed in different laboratories to assess vaccine immunogenicity may generate similar results but critical comparisons will probably require samples to be tested in the same laboratory at the same time.