1. Heat killed organisms of the MS (OH), MR (ØH), NS (O) or NR (Ø) forms of hog cholera bacilli failed to confer immunity, at least to any considerable degree, in hamsters or rabbits against virulent hog cholera bacilli.
2. Hamsters and rabbits were successfully immunized against virulent MS bacilli by vaccination with living MR (ØH) and NR (Ø) forms.
3. Hamsters were immunized against MS hog cholera bacilli with living R paratyphoid "C" bacilli and conversely against MS paratyphoid "C" bacilli with living R hog cholera bacilli.