Parasitism by the braconid wasp Cotesia congregata causes major alterations in the hemolymph proteins of host tobacco hornworm larvae. Earlier studies showed that the total amount of hemolymph protein is reduced during parasitism, beginning almost immediately after the host is parasitized. Simultaneously, parasitism induces synthesis of large amounts of novel proteins that appear in the blood as early as 1-2 h post-parasitization. The present report confirms earlier studies describing the presence of novel proteins in last instar hosts, and also characterizes the effects of parasitism in altering the titers of several endogenous host hemolymph proteins normally produced by the fat body and other tissues. Analysis of hemolymph plasma using SDS-PAGE and densitometry, as well as immunodiffusion assays, showed that in terminal stage fifth instar host larvae, the titers of serpins and arylphorin were dramatically reduced relative to the levels of these proteins detected in nonparasitized gate II fifth instar larvae of the same age. The relative differences between parasitized and nonparasitized larvae increased with time following ecdysis to the fifth instar, so that the day 4 nonparasitized larvae had arylphorin titers of c. 30 mg/ml, whereas parasitized day 4 larvae with newly emerged wasps had only one sixth that amount of storage protein circulating in the hemolymph. Similarly, in nonparasitized larvae the hemolymph serpin concentration increased from c. 200 micrograms/ml (on day 0) to > 600 micrograms/ml (on day 4) in prewandering gate II larvae, but in parasitized larvae the hemolymph serpin concentration was maintained in the range of 100-200 micrograms per ml hemolymph until the pharate third instar parasites emerged from the host larva on day 4. In contrast, the level of hemolymph lipophorin was unaffected by parasitism, and lipophorin increased from c. 1.3 to > 3 mg/ml during the time interval between days 0 and 4 in both nonparasitized and parasitized larvae. Hemolymph titers of insecticyanin also were not significantly different in parasitized vs nonparasitized larvae, and in both types of larvae the concentration of this pigment decreased by c. 50% during the same time interval when lipophorin was increased significantly. Instead of causing a generalized inhibition of host hemolymph protein synthesis, parasitism causes a complex array of changes in the hemolymph protein profile of Manduca sexta, possibly via the mediation of hormonal modulators of host protein synthesis, or transcriptional or translational regulation of host gene expression by factors associated with the polydnavirus or molecules secreted by the parasites.(ABSTRACT TRUNCATED AT 400 WORDS)