The plasma fraction of silkworm (Bombyx mori) hemolymph was fractionated and a preparation (referred to as Sup-II) obtained which retained the prophenoloxidase activating system but did not contain prophenoloxidase, lipophorin or storage proteins. The system in Sup-II was triggered by zymosan and peptidoglycan. When Sup-II, supplemented with purified prophenoloxidase, was triggered by zymosan, prophenoloxidase was transformed into phenoloxidase with a polypeptide molecular weight smaller than that of the proenzyme. Phenoloxidases, activated both by cuticular prophenoloxidase activating enzyme and in Sup-II through the action of zymosan, co-migrated in SDS-PAGE. Phenoloxidase activated by microbial product in Sup-II, formed aggregates which could not be dissociated in the presence of SDS and β-mercaptoethanol. The activity of phenoloxidase seemed to be related to the formation of the aggregates, since aggregates were barely detected in the presence of thiourea (an inhibitor of phenoloxidase activity). Apo-protein I of lipophorin and three other plasma proteins seemed to form covalently linked aggregates through the action of phenoloxidase. The implications of the above findings are discussed in relation to the function of the prophenoloxidase activating system in insect defense.