AbstractAnalyses using one‐dimensional sodium dodecyl sulfate‐polyacrylamide gel electrophoresis (SDS‐PAGE) previously demonstrated that parasitization by the braconid waspCotesia congregatasignificantly alters the normal hemolymph polypeptide profile of hostManduca sextalarvae. In the present study two‐dimensional gel analyses corroborated our earlier findings and provided additional evidence that multiple parasitism‐specific polypeptides were induced, which varied according to the stage of development of the wasps. Parasitization additionally elicited changes in the total protein concentration detected in the blood. Initially an elevation was observed, with newly parasitized larvae exhibiting a twofold elevation in hemolymph protein concentration by 12–24 h postoviposition. In contrast, terminal‐stage hosts with second instar parasites had significantly less protein in the hemolymph, likely due to reduced growth and inhibition of arylphorin synthesis by the fat body during the final stages of parasitism. Comparison of the array of hemolymph polypeptides produced in unparasitized larvae injected with 106cells of the gram‐negative bacteriumEnterobacter cloacaewith those proteins induced by parasitization indicated the two classes are different. Our findings confirm that the hostresponse to parasitism is a specific one, and not mimicked by bacterial challenge. Duringshort‐term in vitro culture of wasp larvae dissected from the host hemocoel, several proteins were detected in the medium using SDS ‐ PAGE, with their appearance in vitro suggestive of secretion by the wasps in vivo. Moreover, hemolymph from the parasites had significant amounts of putative host proteins, including an arylphorin ‐ like polypeptide and a protein with a mobility similar to that of insecticyanin. Thus, a dynamic interchange of proteins may occur, with the parasites accumulating host proteins while simultaneously secreting a variety of factors i