The microagglutination assay is a useful method for the diagnosis of B. pertussis infections. In a group of 30 patients with culture proven pertussis 27 (90%) had > or = fourfold increases in titers between acute and convalescent phase serum specimens. The microagglutination test offers several advantages over other more sophisticated B. pertussis antibody tests: only 50 microliters of serum is required, it is a standardized test, which doesn't require specialized technical expertise or equipment, it is easy to perform and good results are noted in a broad age range of patients. Disadvantages of the microagglutination test are: two separate serum specimens are necessary (acute and convalescent phase), the test does not differentiate IgA and IgG antibodies and the temporal association with recent immunization can lead to false positive results. In our opinion the microagglutination test is a useful method for the diagnosis of B. pertussis infections. This is especially true in cases where more sophisticated serologic tests such as ELISA can not be performed immediately but physicians and patients expect to get a result quickly.