Qualitative and quantitative changes in haemolymph proteins in Heliothis virescens were observed in larvae injected with either Microplitis croceipes teratocytes or teratocyte secreted proteins (TSP). Haemolymph protein titres in hosts receiving either 0.5 or 1 larval equivalent (LE) of teratocytes were similar to those of parasitized larvae, whereas a single injection of 4LE of TSP was required to induce a similar response. SDS-PAGE showed that the 82kDa monomer of riboflavin-binding protein and the 74/76kDa monomers of storage proteins were significantly reduced in parasitized larvae and in nonparasitized larvae treated with TSP. Concentrations of a 155kDa monomer (insectacyanin chromoprotein) also were reduced in parasitized larvae and those injected with either teratocytes or TSP. Two monomers (56 and 60kDa) were unique to parasitized larvae. Treated larvae required several days longer than controls to reach a comparable premetamorphic stage (burrowing-digging). Reductions in fat body proliferation similar to those seen in parasitized larvae were observed in larvae treated with either 1LE of teratocytes, or with 2 or 4LEs of TSP. Perivisceral fat body weights from larvae treated with either 0.25 or 0.5LE of teratocytes were significantly reduced, but less so than those which received 1LE. Thus, fat body proliferation in both teratocyte- and TSP-treated larvae was inhibited in a dose-dependent manner. Both light- and transmission electron microscopy observations revealed cytological differences in fat body tissues of larvae injected with either teratocytes or TSP from the condition observed in parasitized larvae and noninjected controls. Gross dissection of periviseral fat body from parasitized, teratocyte-injected and TSP-injected larvae showed tissue much less developed and differing considerably in appearance from controls. Observed differences included reduced size and/or number of lipid bodies and qualitative and quantitative changes in other cytoplasmic organelles.