Arthropod hemolymph contains proteins with serine proteinase inhibitory activity. These inhibitors may exist in plasma or in hemocyte granules. Serine proteinase inhibitors from the Kazal, Kunitz, alpha-macroglobulin, and serpin families have been identified in arthropod hemolymph and have been characterized biochemically. Two new families of low molecular weight serine proteinase inhibitors have recently been discovered: one in silkworms (the Bombyx family) and another in locusts and a crayfish. The serine proteinase inhibitors in arthropod hemolymph are likely to function in protecting their hosts from infection by pathogens or parasites. Some may inhibit fungal or bacterial proteinases. Others probably have roles in regulating endogenous proteinases involved in coagulation, prophenol oxidase activation, or cytokine activation.