Cells from explants and monolayer subcultures of adult human brain obtained from biopsies or autopsies of ten multiple sclerosis (MS) cases, one case each of Jakob-Creutzfeld (JC) and amyotrophic lateral sclerosis (ALS) and three cases with no central nervous system (CNS) involvement were transformed with PML-SV40 virus. Transformation was effected to establish permanent lines of these particular adult brain cells so that sufficient quantities would be available for other research projects. The procedure previously used to transform human fibroblasts (Koprowski et al., '62) was successfully applied to human brain cells. The success of transformation was dependent on the growth condition of the cultures at the time of infection. Events occurring after viral infection and during the pre-transformation and the post-transformation phases are described.