The brains from 16 sheep diagnosed as listerial encephalitis on the basis of clinical signs and histopathology were examined to determine the types of inflammatory cells present and for bacteria. Listeria monocytogenes were detected by immunocytochemistry in the brains of the 16 sheep and were also demonstrated in the trigeminal ganglion from the same side as the most severely affected side of the medulla. Bacteria were observed within myelinated axons in white matter tracts of the brain and in peripheral nerve axons of the trigeminal nerve. Their location in axoplasm was confirmed by electron microscopy. In order to investigate whether axonal presence can be equated with axonal transport, a model of infection has been established in mice in which bacteria are injected into the sciatic nerve. Paralysis of the injected leg occurs 7-12 days later and examination of the spinal cord reveals lesions very similar to those found in the brains of affected sheep.