Expression of glycoproteins has been carried out successfully using recombinant vaccinia virus vectors. Especially attractive is the use of recombinant vaccinia viruses which express the DNA-dependent RNA polymerase of the phage T7 (T7-polymerase). The T7-polynerase drives the transcription of plasmid-based genes under the control of the T7 RNA polymerase promoter transfected into the infected cell. Comparison of two different recombinant vaccinia viruses, vTF7-3 and MVA-T7, revealed that post-translational processing of Marburg virus surface glycoprotein (GP) is impaired in the MVA-T7 but not in the vTF7-3 system. Influenza virus hemagglutinin, however, was transported and processed like the authentic protein in both systems. It is shown that transport of GP in the MVA-T7 system is not completely blocked, but the vast majority of molecules remained Endo H-sensitive. Only trace amounts evaded the endoplasmatic reticulum and reached the plasma membrane. Thus, the adverse effects of MVA-T7 on the processing of recombinant glycoproteins cannot be predicted, and correct processing has to be investigated for every expressed glycoprotein.