The results of our investigation based upon bacteriologic and virologic findings were, indeed, in keeping with the conclusion that Dr. Macaulay drew from clinical observations. Since 1951, however, many authors have expressed the contrary conclusion that a rise in titer of cold agglutinins is virtually diagnostic of Mycoplasma pneumoniae (Eaton agent) infection. Our observations were considered to be of interest because they failed to support the latter view.