Trichoplusia ni larvae were infected with baculoviruses containing genes for the expression of ultraviolet optimized green fluorescent protein (GFPuv) and several product proteins. A GFP-specific optical probe was used to both excite the green fluorescent protein (lambda(ex) = 385 nm), and subsequently monitor fluorescence emission (lambda(em) = 514 nm) from outside the infected larvae. The probe's photodetector was connected to a voltmeter, which was used to quantify the amount of GFPuv expressed in infected larvae. Voltage readings were significantly higher for infected vs. uninfected larvae and, by Western analysis, linear with the amount of GFPuv produced. In addition, the probe sensitivity and range were sufficient to delineate infection efficiency and recombinant protein production for model proteins, chloramphenicol acetyltransferase and human interleukin-2. This work represents a critical step in developing an automated process for the production of recombinant proteins in insect larvae. (C) 2003 Wiley Periodicals, Inc.