We have recently described a novel, stable human immunodeficiency virus type 1 (HIV-1) vector packaging system, STAR. High-titre HIV-1 vectors bearing gammaretrovirus envelopes (Env) are continuously produced from STAR cells. Here we compare the properties of such vectors, with the amphotropic murine leukaemia virus (MLV-A) Env, a modified gibbon ape leukaemia virus (GALV) Env and two modified versions of the cat endogenous retrovirus RD114 Env, produced from STAR cells, to transiently produced HIV-1 vectors with vesicular stomatitis virus G protein (VSV-G). Our results indicate that gammaretrovirus pseudotypes from STAR cells are relatively stable at 37 degrees C and are resistant to inactivation by freeze/thaw cycling or incubation with human sera. HIV-1(VSV-G) was, however, sensitive to freeze/thaw when harvested in serum-free media and was readily inactivated in human sera. Furthermore, the titre of 'gamma-retrovirus' pseudotypes, but not HIV-1(VSV-G), could be increased by the use of a combination of polybrene and spinoculation. All pseudotypes could be efficiently concentrated, but soluble gammaretrovirus Env could act as an inhibitor of infection.