Radioactively labelled virus particles of intracellular origin were isolated from the cytoplasmic fraction of disrupted NIH/3T3 cells chronically infected with Moloney murine leukaemia virus [NIH/3T3 (MLV)]. Interferon (IFN) treatment for 48 h, which arrested more than 90% of virus release, resulted in a remarkable accumulation of these intracellular virions. However, no major effect of such treatment was apparent on their structural properties. Transmission electron microscopic examination revealed that these intracellular virions were located within cytoplasmic vacuoles. IFN treatment resulted in a considerable increase in the number of virus-containing vacuoles, as well as the total number of vacuolar virions. It seems that IFN inhibits the final release of vacuolar virions from the cells, thus leading to their intracellular accumulation.