SummaryHA reduction assays for IF were performed using rapid (10 sec) Tween 80-ether extractions of Sindbis virus yields to greatly amplify measurable HA activity. Employing this technique a sensitive HA reduction assay for IF was developed for use in a cell system (human amnion U cells) that produced insufficient viral HA particles for measuring IF concentrations by normal HA reduction assays. When this extraction technique was employed to amplify HA yields in a system where large quantities of virus were produced (chick embryo cells), significant increases in sensitivity and precision of IF assays were observed. It appears possible that this technique may have wide application in IF assays employing other cell systems and viruses.