The in vitro transformation of lymphocytes stimulated by a Mycoplasma pneumoniae preparation was measured by the uptake of 14C-thymidine. The lymphocytes from five patients with M.pneumoniae pneumonia showed a high degree of responsiveness when they were compared to the lymphocytes taken from eleven healthy control subjects who lacked M.pneumoniae antibodies. Another four patients with an acute affection of the central nervous system and serological evidence of an actual or recent M.pneumoniae infection had a lymphocyte response within the same range as that of the controls. The transformation of lymphocytes was studied at intervals for seven months after the onset of the illness in one of the patients with pneumonia. These studies showed an increasing response to a small dose of mycoplasma antigen. Lymphocyte transformation induced by other microbial antigens was studied in three pneumonia patients during and after convalescence. The first responses were low and increased more steeply than the response to M.pneumoniae. The later responses to the mycoplasmal and to the other microbial antigens increased in parallel. The usefulness of incorporating other microbial antigens in the evaluation of the patient's immune response to a relevant antigen in this type of experiment is discussed.