Antibodies in sera and respiratory secretions from chickens infected with Mycoplasma gallisepticum (MG) were measured by an enzyme-linked immunosorbent assay (ELISA). Chickens intratracheally inoculated with 10(5) cells of MG showed a correlation between severity of tracheal lesions and extent of MG colonization in the tracheas in the first 3 weeks postinoculation. Antibody titers in tracheal washings (TWs) of the infected chickens increased during this phase. Thereafter, isolation of MG from the trachea decreased sharply, and there was a concomitant decrease in tracheal lesion scores. At 5 weeks postinfection, the chickens that recovered from the infection exhibited a consistent presence of antibodies in TWs. Chickens reexposed had a faster rate of MG elimination and substantially less severe inflammatory lesions in the tracheas than chickens observed after the first exposure. These findings suggest a possible role of antibodies of the respiratory secretions in resistance to MG. The ELISA was a sensitive and reliable test to detect a minute amount of antibodies in the secretions.