Antibody titer, lymphocyte stimulation and leukocyte migration inhibition with chlamydial antigens were determined repeatedly over many months on human subjects. The volunteers were retrospectively placed into four groups on the basis of clinical, laboratory and epidemiologic criteria. Group A consisted of persons with proven or probable chlamydial infection, including an illness confirmed by chlamydial isolation or seroconversion, or a clinically compatible illness with positive serologic results. Group B were sexual partners or close contacts of group A individuals. Group C were laboratory workers with prolonged exposure to viable chlamydiae or their antigens. Group D included persons of comparable age as those in groups A and B, but lacking a history of symptomatic chlamydial infection or of contact with chlamydiae. Individual cases illustrated the rise of antibody and some cell mediated immunity reactions (CMI) with active chlamydial infection. By contrast, laboratory exposure resulted in elevation of CMI but not of antibody. Statistical analysis of the results in 46 volunteers tested repeatedly indicated a strong association of specific antibody with lymphocyte stimulation, but not with leukocyte migration inhibition. Regression analysis suggested that the type of exposure markedly influenced the relationship between antibody and lymphocyte stimulation. Measurement of immunotype-specific antibody titer by microimmunofluorescence (or an equally sensitive method) remains the best laboratory indicator of past chlamydial infection. Neither antibody nor CMI can, as yet, be definitely related to resistance to re-infection in humans.