We investigated monitoring for baculovirus infection of insect Trichoplusia ni larvae by combining the green fluorescent protein (GFP) reporter, the early-to-late (ETL) baculoviral promoter, and a GFP-specific optical probe. Because GFP is under the ETL promoter control, it facilitated rapid monitoring of recombinant baculovirus infection on insect larva system. Employment of GFP-specific optical probe also enabled non-invasive and accurate GFP monitoring with similar to 10 h prior to invasive and GFP Western blot assay. This combination may assist in monitoring protein production runs, determining optimal infection and harvest timing, and in general, help to increase the yield of recombinant proteins expressed in larvae. (c) 2005 Elsevier Ltd. All rights reserved.