Printed in Great Britain 32I Host Range Conversion of the Murine Sarcoma-leukaemia Complex

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 loo- to IOOO-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