The antigenic specificity of the human immune response during legionellosis was studied. Twenty-two sera obtained from 12 individuals were studied using the Western-blot technique with antigens from various legionella. In five of six patients, the reactivity of their sera obtained at different times during legionellosis was qualitatively constant. In one patient, a new IgG antibody to a 50 Kdal bacterial antigen appeared between the 14th and 22nd day after infection. In every patient, the IgG and IgM responses differed. When the patient's serum was reacted with antigen prepared from the isolate obtained from that patient or from a closely related isolate, the antibody response appeared to be directed against broad antigenic bands.