, Stephanie Andriole , Kurt

tion method reported by Sussman (5) and practically sufficient to determine the inoculum size of baculovirus for plaque isolation or large-scale protein expression. Based on these observations, we replaced the plaque formation assay with the β-gal activity assay to obtain viral titers in our routine laboratory practice. We have successfully shortened the periods of time from transfection to preparation of the recombinant proteins in large-scale insect cell suspension cultures. When the baculovirus was concentrated by liquid chromatography as described by Barsoum (1), the titer determined by plaque assay did not show the correlation with the titer estimated by the β-gal activity assay (data not shown) because our method is not associated with the viral particle itself. Therefore, our method is not applicable for determining viral stock titer after concentration. When the virus preparations were stored at 4°C, both the plaque formation assay and the β-gal activity showed no significant decrease for at least six months (data not shown). For an accurate estimation, we strongly recommend including a standard virus preparation of known titer in the β-gal activity assay to construct a standard curve for each assay. REFERENCES