Microbiologic tests are essential in the diagnosis and management of patients with syphilis. Apart from the very early stage of disease (when T. pallidum may be detected in the lesions of primary syphilis before an antibody response is detectable) serology is the mainstay of laboratory testing. The performance of cardiolipin antigen ("reagin") and treponemal antigen (native and recombinant) tests is discussed in relation to the stage of syphilis, treatment status, and interactions between syphilis and HIV infection. Screening with cardiolipin antigen tests detects early stage disease, but treponemal antigen tests are required for the reliable detection of late-stage infection and the exclusion of syphilis in HIV-infected patients. EIA tests using treponemal antigen are sensitive and specific and fit well into current laboratory practices. Although the FTA-abs test is often considered the "gold standard" confirmatory test, its sensitivity is slightly lower than that of certain other treponemal antigen tests. A reactive antitreponemal IgM test correlates well with untreated or recently treated early infection, but specific IgM tests are often negative in untreated late-stage disease. Serial quantitative cardiolipin antigen tests remain the method of choice for monitoring the efficacy of treatment. The role of the laboratory in aiding a diagnosis of neurosyphilis and congenital infection is discussed briefly, as is the current status of newer technologies, such as PCR and immunoblotting.