DEVELOPMENTAL AND COMPARATIVE IMMUNOLOGY, Vol. Ii, pp. 479-485, 1987. 0145-305X87 $3.00 + .00 Printed in the USA. Copyright (c) 1987 Pergamon Journals Ltd. All rights reserved. THE MAJORITY OF PROPHENOLOXIDASE IN THE HEMOLYMPH OF MANDUCA SEXTA IS PRESENT IN THE PLASMA AND NOT IN THE HEMOCYTES. Steven J. Saul, Liu Bin and Manickam Sugumaran Department of Biology University of Massachusetts at Boston Harbor Campus, Boston, MA 02125 INTRODUCTION The importance of phenoloxidase in protection of insects against invading microorganisms and parasites is well documented (1-3). Phenoloxidase is usually present in insect hemolymph as a proenzyme form and is activated by limited proteolysis (4-7). The active enzyme catalyzes the deposition of melanin pigments around the foreign objects. Accordingly, observed prophenoloxidase activation mediated by bacterial cell wall preparations and ~-l,3-glucans is consistent with its participation in defense reactions of insects (8-10). Given such a key function, it is important to know the localization of this enzyme in the hemolymph. Earlier studies by Pye (9) as well as Evans (Ii) indicated the presence of this enzyme in the hemolymph plasma of Galleria mellonella and Antheraea pernyi larvae. Similar results have been reported by Ashida with silkworm Bombyx mori larvae also (I0). On the other hand, Leonard, et al. (12) have recently claimed that this enzyme is mainly localized in the hemocytes and not in the plasma of Blaberus craniifer. Our laboratory has been studying the control mechanisms involved in the activation of prophenoloxidase in Manduca sexta (13-15). Hence, we examined the compartmentalization of this enzyme in the hemolymph of fifth instar larvae and report in this communication, that the majority of prophenoloxidase is present in the plasma and only a minor fraction in the hemocytes of Manduca sexta. *Person to whom correspondence should be addressed. 479