Neutralising antibody was studied in mice infected in the ear with herpes simplex virus type 1. Antibody was detected six days after infection and after one month it had reached titres which subsequently varied little. Geometric group mean titres were doubled by reinoculation of virus whereas recurrent disease induced by trauma did not change them (although titres did change in some individual animals). Mean titres in mice which had had unequivocal signs of primary disease were twice as high as in those which had not, and the frequency of recurrent disease in response to trauma in the former animals was greater than in the latter. A reduction in the frequency of induced recurrent disease was seen if the application of trauma was delayed for some time after the establishment of latency. Hyperimmunisation increased mean titres tenfold. If given soon after primary infection it reduced the frequency of induced recurrent disease but if its administration was delayed then the frequency was increased.