It has been suggested in the past that Praomys (Mastomys) natalensis might be an animal model for human myasthenia gravis. This suggestion was based on the occurrence of thymomas and autoantibody to striated muscle in this animal species. Myasthenia gravis in man is associated with anti-striated muscle antibody and thymoma as well as antiacetylcholine receptor antibody. This prompted us to search for such autoantibodies in Mastomys. There was no evidence of anti-acetylcholine receptor antibody in any of the serum samples tested. The titres found in Mastomys correspond to those observed in human control sera. No difference was found in the number of alpha-bungarotoxin-binding sites at the motor endplate and in muscle extracts between animals with and without thymoma and with and without anti-striated muscle antibody. These findings lead to the conclusion that it is very unlikely that myasthenia gravis occurs with any frequency in Praomys (Mastomys) natalensis.