The matrix (M) protein of vesicular stomatitis virus (VSV) is a multi-functional protein involved in virus assembly, budding and pathogenesis. The (24)PPPY(27) late (L) domain of the M protein plays a key role in virus budding, whereas amino acids downstream of the PPPY motif contribute to host protein shut-off and pathogenesis. Using a panel of (37)PSAP(40) recombinant viruses, it has been demonstrated previously that the PSAP region of M does not possess L-domain activity similar to that of PPPY in BHK-21 cells. This study reports the unanticipated finding that these PSAP recombinants were attenuated in cell culture and in mice compared with control viruses. Indeed, PSAP recombinant viruses exhibited a small-plaque phenotype, reduced CPE, reduced levels of activated caspase-3, enhanced production of IFN-beta and reduced titres in the lungs and brains of infected mice. In particular, recombinant virus M6PY>A4-R34E was the most severely attenuated, exhibiting little or no CPE in cell culture and undetectable titres in the lungs and brains of infected mice. These findings indicate an important role for the PSAP region (aa 33-44) of the M protein in the pathology of VSV infection and may have implications for the development of VSV as a vaccine and/or oncolytic vector.