Studies attempting to resolve the mechanism of lepidopteran-specific Bacillus thuringiensis toxicity initially were hampered by a lack of in vitro systems, which are necessary to define the reaction in molecular terms. Insect cell lysis in vitro was first documented by Murphy et al., using Trichoplusia ni gut fluid-activated protoxin from Bacillus thuringiensis subsp. kurstaki. There have been excellent reviews up to and including work published in 1988 documenting in vitro studies with Bacillus thuringiensis toxic proteins using cells in culture. There has been one in vitro study examining Bacillus thuringienis resistance in vitro, and this experiment involved an attempt to select a population of cytolytic to lepidopteran cells, cells resistant to Bacillus thuringienis subsp. Although important information on various aspects of Bacillus thuringienis toxin biochemistry and mode of action has been obtained through the use of established insect cell lines, a review of the recent literature indicates a decline in studies with lepidopteran cell lines.