The cellular, humoral, and local immune responses of chickens to Mycoplasma gallisepticum infection were studied at weekly intervals for 10 weeks. A cellular response was indicated by significant leukocyte migration inhibition (LMI) was demonstrated as early as one week postinfection (PI), but the maximum LMI of 36.4% was observed at seven weeks PI. Induction of cellular response was further confirmed by positive-delayed type of hypersensitivity reaction, observed in all infected birds tested. The antibody response, as determined by the hemagglutination-inhibition (HI) test, was found to be provoked very early in infection, and the mean HI titer peaked seven weeks PI. A highly significant correlation (p less than 0.05) was found between LMI percentages and mean HI titers. In the infected chickens a significant elevation of mean serum immunoglobulin (Ig) G level was noted from the first week PI onwards; this correlated well with the rise of HI antibodies. There was no change in the serum IgA and IgM levels. A low level of M. gallisepticum-specific HI activity was detected in tracheal washings (TW) of infected birds from the third week PI onwards; the levels peaked six weeks PI. The IgA/IgG ratio in TW of infected chickens did not differ significantly from those of controls. The recovery of chickens as assessed by regression of lesions and disappearance of organisms from trachea coincided with peak LMI, maximum HI titers in serum and TW, and elevated levels of serum IgG.