The serodiagnostic methods commonly used for the detection of antibodies in various chlamydial infections, the value of serodiagnosis, and the role of humoral responses in human immunity are reviewed. The methods most commonly used for serodiagnosis of chlamydial infections are indirect immunofluorescence tests and enzyme-linked immunosorbent assays. These tests are highly sensitive and have been extensively used in epidemiologic and clinical studies of chlamydial infections. The evidence suggests that these tests are cost effective for population screening and that the presence of a high level of specific IgG or IgM in blood or of IgG or IgA in local discharges is a useful indicator for the provisional diagnosis of chlamydial infection.