The experiments recorded in the present paper confirm the existence of 4 forms of the hog cholera bacillus described in a previous paper (1), namely the "normal" type strain MS and its 3 variants, MR, NS and NR. Serological evidence is also presented to show that the symbols MS, MR, NS and NR represent the similar conceptions of previous investigators who have used the letters OH, O, ØH and Ø respectively to designate variant forms of other organisms. It has been shown that the S form of the hog cholera bacillus, as the S forms of other bacteria, is more virulent for laboratory animals than the R forms.

In regard to the reversibility of one form to another it was found that by transfer in broth or by passage through mice the MR form showed a tendency to revert to the original MS form. Transfer in broth and animal passage, however, failed to induce any variations in the NR or NS forms. Growth in homologous immune sera did not cause reversion to original forms but in fact provoked further dissociation of the MR to the NR form and also of the NS to the NR form. The MR form may revert to the original normal MS form or may dissociate further into the NR form and is, therefore, the most unstable of the variants. Further attempts to induce changes in the other variants were not made. The dissociation as observed may be represented thus:

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