The effects of interferon treatment on the expression of murine mammary tumour virus (MuMTV) by a continuous mammary tumour cell line of GR mouse were investigated. Interferon markedly inhibited the excretion of MuMTV particles in the culture media as measured by hybridization of virus-specific RNA and virus-associated reverse transcriptase activity. There was no inhibition of virus RNA and protein synthesis in those conditions. The steady-state level of intracellular virus RNA and its rate of synthesis were not modified by interferon treatment. The intracellular levels of the major virus envelope and core proteins as measured by immunoprecipitation techniques remained unchanged. Interferon failed to inhibit the synthesis and the processing of the gp52 glycoprotein and p27 core protein precursors. However, the rate of maturation of the glycoprotein precursor was slowed down. Surface labelling of intact cells did not reveal accumulation of virus proteins on the cell membrane upon interferon treatment. The intracellular level of MuMTV-characteristic reverse transcriptase activity was reduced in interferon treated cells, although to a lower extent than extracellular virion-associated enzyme activity. MuMTV particles released from interferon-treated cells revealed no difference in their protein composition. These results are consistent with an inhibition by interferon of a late step in the replication of MuMTV.