Morphological changes were induced by cis-dichlorodiammine platinum (II) (cis-DDP) in two Epstein-Barr virus (EBV) transformed cell lines: the productive P3HR-1 and the nonproductive Raji cell line. In P3HR-1 cells cis-DDP induced synthesis of viral antigens, viral particles and morphological changes characteristic for virus replication. In Raji cells, the virus replicative cycle was not induced and virus-specific morphological changes were limited to the sporadic appereance of some very early alterations in cell morphology. However, in Raji cells, but not in P3HR-1 cells, up to 20 per cent of the cell population exhibited differentiation-related changes towards plasma cell morphology. The most advances stage of cell differentiation detected was classified as plasmablast. Cis-DDP also induced some changes associated with the cytostatic effect of the drug. In the treated cell population cytokinesis was inhibited and frequently multinuclear cells appeared; moreover, extensive degenerative changes were observed.