The protein composition of larval and adult hemolymph from the Colorado potato beetle, Leptinotarsa decemlineata, was investigated and some abundant, high molecular weight proteins were identified and characterized. Diapause protein 1, which occurs in the hemolymph of last instar larvae and short-day adults, appeared to be a storage protein. This protein dissociated into two bands due to the high pH used in nondenaturing gels. Its quaternary structure was established by chemical crosslinking. It appeared to be a hexamer. Diapause protein 1 is composed of approximately 82,000 subunits. The amino acid composition and N-terminal sequence of this protein has been determined. Specific antibodies against diapause protein 1 have been developed. Topical application of 1 microgram pyriproxyfen, a juvenile hormone analog, to last instar larvae and short-day adults suppressed the appearance of this protein in the hemolymph. Pyriproxyfen prematurely induced vitellogenin, when applied to last instar larvae. A larval specific protein was also identified in the hemolymph. Its temporary appearance in the hemolymph of last instar larvae, its subunit composition (M(r) approximately 82,000) and its suppression by pyriproxyfen suggests that this protein is a storage protein as well.