The juvenile hormones (JH) play a central role in the development and reproduction of insects. Although biological data overwhelmingly demonstrate that the hormones direct differentiation, the biochemical and subcellular actions of the hormones are unclear. In an effort to better understand the mode of action of the juvenile hormones in immature larvae, we have identified and characterized a chromoprotein in the hemolymph and epidermis of Manduca sexta which is extremely sensitive to JH. To study this response, we have developed a sensitive and specific immunoassay for this marker protein. Employing this marker and a mutant strain which lacks the ability to store and/or synthesize the protein in the absence of JH, we can probe the molecular action of JH on the epidermal cell. The effect of JH on packaging or discharge of the chromoprotein, insecticyanin, will be investigated on two levels. JH may initiate postranslational events causing the modified protein to be stored in intracellular granules. Unmodified insecticyanin is presumably secreted into the hemolymph. JH may direct intracellular protein traffic via cytoskeletal elements. Changes in cytoskeletal structure may be responsible for retaining insecticyanin-containing granules or permitting the movement of the chromoprotein to the basement membrane for release.