First, we sought to measure the kinetics of content transfer over a population of viruses and vesicles of variable sizes. Unilamellar vesicles (100 nm or 200 nm ULVs) encapsulating QuantiFluor were prepared and tethered to the surface of microfluidic flow cells. We added the influenza virus particles and allowed the viruses to bind to the vesicles. Next, the pH was dropped to 5.1 and bright spots corresponding to fusion events were detected. The spots were followed by continuous imaging and intensity traces were extracted from those videos (Figure , Figure ). The time point (wait time) where we observed a sudden spike in intensity corresponded to the onset of the fusion event. Those wait times were then combined into cumulative distribution functions (CDF). From the CDFs, we observed that the content transfer kinetics of viruses with 100 nm was slightly faster than with 200 nm vesicles (Figure ). For comparison, we also carried out lipid mixing experiments between virus labeled with self-quenching concentration of Texas Red-DHPE and unlabeled 100 nm or 200 nm vesicles. In the event of fusion (or hemifusion), Texas Red-DHPE underwent dilution and a sudden spike in fluorescence was observed in the intensity traces (Figure ). We calculated the waiting time between pH drop and appearance of a fluorescent spot and constructed CDFs to measure kinetics of lipid mixing (Figure ). In good agreement with previous results , we found that lipid mixing took place at a more rapid rate as compared to that of content transfer. Next we estimated what fraction of virus particles undergoing lipid mixing also underwent content transfer. In this experiment, the influenza virus particles were labeled with a selfquenching concentration of Texas Red-DHPE, and fusion was carried out with QuantiFluorcontaining vesicles. We identified the spots which underwent an increase in fluorescence in the Texas Red channel and also co-localized with a corresponding spot in the QuantiFluor channel (Figure ). We estimated that about 40% (40 out of 99) of all vesicles undergoing lipid mixing underwent content transfer. It is notable here, however, that some vesicles may have too few or no trapped dye molecules, so that when content transfer took place to those vesicles, no signal would have been detectable. So, it is possible that the actual fraction of the virus-vesicle pairs undergoing content transfer may be somewhat underestimated.