The induction of proteins in the tobacco hornworm larvae, Manduca sexta, and the adult tropical cock- roach, Blaberus giganteus, was examined after chal- lenge with cell walls of the entomopathogenic fungus Metarhizium anisopliae or the yeast, Saccharomyces cerevisiae. This was compared to induction of proteins after challenge with �-1,3-glucan or cell walls from the bacterium Micrococcus lysodeikticus. Induction of pro- teins was examined in live insects, in vitro cultures of fat bodies excised from insects, or an embryonic cell line from M. sexta, MRRL-CH-1. Challenge of fifth instar larvae by injection with S. cerevisiae resulted in the induction of 14- and 33-kDa proteins after 24 hr. Challenge of fat bodies with M. anisopliae cell walls resulted in the induction of several proteins which were not induced after challenge with �-1,3-glucan or M. lysodeikticus. These inducible proteins did not im- munoreact with M. sexta alaserpin or M13 (hemolin) antibodies. However, one of the proteins (ca. 33 kDa) induced in M. sexta by S. cerevisiae, �-1,3-glucan, M. anisopliae cell walls, or cell walls of M. lysodeikticus immunoreacted with antibodies for scolexin. Over 300 protein species were synthesized by B. giganteus fat bodies after a 6-hr incubation and approximately 10 novel proteins were observed after challenge with fungal cell walls. M. sexta cell cultures also responded to chal- lenge with fungal cell walls by synthesizing several novel proteins. The results suggest that holometabolous and hemimetabolous insects synthesize novel proteins in re- sponse to fungal challenge and that sites of synthesis may be in different cell types (fat bodies or epidermal cells).