Summary Measles virus was isolated from brain cell tissue cultures derived from two SSPE patients. These cultures proved to contain intracellular measles antigen which was not released in the fluid phase. Infectious, complete virus was obtained when mixed cultures containing the brain cells and HeLa cells were prepared. It appears that SSPE is due to suppressed measles virus infection. Once “rescued” through the mixed culture technique, the virus recovered from SSPE patients proved indistinguishable from measles virus. Note added in proof: At the time of galley review of this paper, confirmation of our previous isolation of complete infectious measles virus was reported by Payne, F. E., Baublis, J. V. and Itabashi, H. H. (Isolation of Measles Virus from Cell Cultures of Brain from a Patient with Subacute Sclerosing Panencephalitis, New England Journal of Medicine 281, 585, 1969). These authors used the mixed culture technique which we reported for this purpose (Horta-Barbosa, L., Fuccillo, D. A., Sever, J. L. and Zeman, W., Subacute Sclerosing Panencephalitis: Isolation of Measles Virus from a Brain Biopsy, Nature 221, 974, March 8, 1969) with BSC-1 cells and continued propagation of the patient's brain cells. The authors gratefully acknowledge the valuable technical assistance of Miss Rebecca Schronce and Mrs. Anna Barbara Wittig.