Abstract—Harmine, a β‐carboline alkaloid, was found to be an efficient photoactive agent against the DNA‐containing murine cytomegalovirus (MCMV) and the RNA containing Sindbis virus, a togavirus, both of which contain membranes. In contrast the host cells themselves were relatively resistant. The MCMV was investigated in more detail. Virus which had been inactivated by harmine plus UVA retained its structural integrity and was fully capable of penetrating host cells, following which the viral genome entered the cell nucleus. The viral genes which normally code for the switch‐off in thymidine kinase activity arid for the normal cytopathiceffectswere not expressed; however the viral geneis responsible for the cytotoxic effect due to high multiplicities of infection was still expressed, indicating that there may be a differential block of some vi