Abstract 1. 1. Using immunoelectrophoresis at least eleven antigens were detected in hemolymph from fourth-instar nymphs, from male and female fifth-instar nymphs 1–8 days post-molt, from male and female adults 1–14 days post-molt, and in the egg. A minimun of five and as many as ten antigens were observed in preparation of first-, second- or third-instar nymphs. Fusion was observed between corresponding precipitin arcs containing antigens from insects of both sexes and all physiological ages studied as well as from the egg. 2. 2. Five of the precipitin arcs gave an esterase reaction which was inhibited by 10 −4 M eserine. 3. 3. Two of the precipitin arcs which stained for esterase also gave a tyrosinase reaction. 4. 4. One of the tyrosine and esterase positive arcs bound-dye specific for lipoprotein and also reacted as a glycoprotein. 5. 5. Some of the protein present bound evans blue, but it did not have the mobility characteristic of human serum albumin. 6. 6. Although the protein concentrations do change during development, the data support the idea that the proteins of the milkweed bugs in different developmental stages do not differ qualitatively in a significant manner.