A 46-year-old female infected with Mycoplasma pneumoniae developed severe pneumonia with hypoxia and anemia; both conditions responded promptly to erythromycin. The patient’s serum contained cold agglutinins at a high titer for 18 days; the agglutinins were polyclonal, cold agglutinin idiotypenegative, and predominantly blood group I-specific. During the acute phase of illness, the patient was anergic to tuberculin. It is suggested that, in addition to hemolysis, bone marrow suppression played an important role in the pathogenesis of the anemia in our patient.