The three classical papers published in 1944 and 1945 by Monroe A. Eaton and colleagues deal with the etiology of primary atypical pneumonia (PAP) and with the properties of a filterable agent (subsequently and for a number of years known as Eaton agent) from the sputum or lung of patients with PAP using cotton rats, hamsters, and chick embryos as laboratory hosts. The present review is first and foremost a tribute to Monroe Eaton and his colleagues for their trail-blazing discovery of a major cause of the atypical pneumonia syndrome and their steadfast vision of its importance. The organism was finally identified and designated Mycoplasma pneumoniae some 20 years after their papers first appeared in the Journal of Experimental Medicine.