the present study we investi-gated the prop0 activating system of the pe-naeid Penaeus paulensis, focusing on its role in the shrimp immune system. The great ma-jority of PO activity (more than 90%) was found in the shrimp hemocytes. The enzyme activity was greatly enhanced by components of microorganism cell walls, such as LPS and f&1,3-glucans, suggesting its involvement in non-self recognition.PO activity was also found in the shrimp serum and trypsin, and LPS were able to increase the enzyme activity. Thus, serum can be used as an alternative for the study of the shrimp prop0 activating system, as it is much more readily obtained than HLS. PO activity was cation-dependent, and 5 mM of calcium and 10 mM of magne-sium were the optimal concentrations for the enzyme activity. An immune factor was found in the shrimp HLS, capable of inducing cell-adhesion and degranulation of the pe- naeid hemocytes. 0 1997 Elsevier Science Ltd.