BACKGROUND: Measles virus (MV) is a member of the Paramyxoviridae family and an 
important human pathogen causing strong immunosuppression in affected 
individuals and a considerable number of deaths worldwide. Currently, measles is 
a re-emerging disease in developed countries. MV is usually quantified in 
infectious units as determined by limiting dilution and counting of plaque 
forming unit either directly (PFU method) or indirectly from random distribution 
in microwells (TCID50 method). Both methods are time-consuming (up to several 
days), cumbersome and, in the case of the PFU assay, possibly operator 
dependent.
METHODS/FINDINGS: A rapid, optimized, accurate, and reliable technique for 
titration of measles virus was developed based on the detection of virus 
infected cells by flow cytometry, single round of infection and titer 
calculation according to the Poisson's law. The kinetics follow up of the number 
of infected cells after infection with serial dilutions of a virus allowed 
estimation of the duration of the replication cycle, and consequently, the 
optimal infection time. The assay was set up to quantify measles virus, 
vesicular stomatitis virus (VSV), and human immunodeficiency virus type 1 
(HIV-1) using antibody labeling of viral glycoprotein, virus encoded fluorescent 
reporter protein and an inducible fluorescent-reporter cell line, respectively.
CONCLUSION: Overall, performing the assay takes only 24-30 hours for MV strains, 
12 hours for VSV, and 52 hours for HIV-1. The step-by-step procedure we have set 
up can be, in principle, applicable to accurately quantify any virus including 
lentiviral vectors, provided that a virus encoded gene product can be detected 
by flow cytometry.