1. Ten pleuropneumonia-like strains isolated from various sources have been selected and studied with a view to discovering reliable criteria for differentiation. Three of them were isolated from the symbiotic association known in the literature as Streptobacillus moniliformis, six from lung lesions of rats and one from a swollen submaxillary gland of a rat. For comparison with these, six other strains, viz. three pleuropneumonia, one agalactia and two of Shoetensack's strains from lungs of dogs stated by him to be suffering from distemper and named by him Asterococcus canis type I and A. canis type II have been included in the inquiry.

2. Classification of the sixteen strains has been achieved by a comparative study of type of growth on liquid and solid media, morphology, pathogenicity and serological affinities. Table VII displays these differences.

3. The serological examination was of special importance for the whole problem of classification. For this purpose an agglutination test was devised which clearly revealed that the sixteen strains belonged to seven different serological types. As the strains of the same antigenic structure showed similar growth types in liquid and solid media and similar morphological appearances, seven species of pleuropneumonia-like organisms were set up:

(a) The L1 organism represents the first species. Three of the strains of this type were separated from Streptobacillus moniliformis cultures. The fourth culture of the same type was found in the lung of a rat, thus proving that this organism may occur independently as well as in symbiotic association.

New evidence of the symbiotic nature of the Str. moniliformis (an association of a streptobacillus with an L1) was given by the agglutination test, in so far as all sera prepared by immunization with Str. moniliformis cultures agglutinated to a fairly high titre the four different L1 strains.

(b) The L3 organism represents the second distinct type species. The five strains belonging to it originate from lung lesions of rats, viz. four from tame laboratory rats and one from a wild rat. This demonstrates that tame and wild rats contain the same variety of pleuropneumonia-like organism in the associated lung lesion. It is believed that 138 pleuropneumonia-like cultures isolated from lung lesions of rats by the writer belong to the same L3 species though only five of them have been studied here in detail.

(c) The L4 organism only once isolated from the enlarged submaxillary gland of a rat also suffering from lung lesions was proved to be definitely different from the L1 and L3 organisms by serological and other tests and represents therefore a third new type species.

(d) Furthermore the organisms of pleuropneumonia, agalactia, Asterococcus canis I and A. canis II would appear to be independent species of the same family.

4. Experiments on pathogenicity which are still in progress show so far that the L3 organism is pathogenic for mice. The L1 organism would appear to be devoid of pathogenicity when tested by similar methods.

5. The capacity of pleuropneumonia-like strains for forming associations with bacteria has been tested experimentally, and in a few cases success has been achieved. These associations with B. tetani and B. tetanomorphus, unlike that of L1 and the streptobacillus in the Str. moniliformis combination, though stable, are not such as to preclude the isolation in pure culture of the two anaerobic bacilli.

6. It is likely that the group of pleuropneumonia-like organisms is widely spread in nature. Search for such strains and thorough investigation of their properties would seem to be of prime importance in view of the minute size of the units which can give rise to growth and the light that might be shed by their study on the biological nature of the filterable viruses.