To determine the time course of production of infectious MARV during a prolonged infection period, supernatants of HUH-7 cells, infected with MARV under BSL-4 conditions, were collected from day one to four post infection (p.i.) and tested for viral protein content, specific infectivity and virion morphology. The amounts of viral NP and VP40 released from the cells were measured by quantitative immunoblotting. This showed that viral protein release peaked between day one and two p.i. ( Figure 1A ). The TCID 50 of each supernatant was determined and normalized to the amount of released NP to estimate the specific infectivity per virus ( Figure 1B , grey areas). Specific infectivity was also at a maximum between day one and two p.i.. Virus morphology in the supernatants was monitored by EM and revealed that at the peak of viral protein production and specific infectivity, 80% of the virus particles displayed the characteristic filamentous morphology of the filoviruses, with the remainder appearing bent or round ( Figure 1B) . Later in infection, the specific infectivity was lower and correlated with fewer filamentous particles and more round or bent particles ( Figure 1B) , suggesting that infectivity can predominantly be attributed to filamentous virus. There was also an increase in the release of cellular vesicular material over time (Table 1) .