Infection by N- or B-tropic murine sarcoma virus (MSV) ‘pseudotypes’ in the presence of murine leukaemia virus (MLV) of the opposite tropism to non-permissive cells of the MSV resulted in about a 100- to 1000-fold increase of focus formation in comparison with MSV solitary infection. The culture fluid of such mixedly infected cells contained MSV ‘pseudotypes’ whose tropism was identical to that of MLV.