White rabbits were immunized with pooled and concentrated cerebro-spinal fluid (CSF) and with tumour homogenate concentrate from specimens taken from five patients with acoustic neurinoma. The absorbed anti-CSF-antitumour antiserum was tested with the micro-immunodiffusion test against normal human serum (NHS), CSF, perilymph and CSF from tumour patients and tumour homogenate. NHS showed no precipitates in any of the tests. One protein band was observed in all the other four reactants. Tumour tissue and perilymph had two proteins in common. Diffusion from the CSF space into the perilymph can thus occur via both the cochlear aqueduct and the internal acoustic meatus.