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IN VITRO AND IN VIVO RESPONSES OF CIRCULATING HEMOCYTES IN THE APPLE SNAIL Pomacea canaliculata (ARCHITAENIOGLOSSA, AMPULLARIIDAE) J. A. CUETO, C. FADER, H. H. ORTEGA and A. CASTRO-VAZQUEZ. Laboratorios de Fisiología (IHEM, CONICET-UNCuyo) y de Investigaciones Histológicas Aplicadas (UNL), Argentina. E-mail: cueto.juan@fcm.uncu.edu.ar. Pomacea canaliculata (Lamarck 1822) shows a circulating hemocyte concentration of about 3000 cells/µL, 90% of which are hyalinocytes whose cytoplasm is packed with vesicles of the smooth endoplasmic reticulum and it also shows some granules of moderate electron density. These cells attach to the glass surface, emit pseudopodia and aggregate between them in a Ca2+ containing medium. Heat-inactivated bacterial and yeast cells are phagocytosed by hemocytes in those conditions, while Ca2+ chelation by EDTA results in inhibition of all these hemocyte reactions. Intramuscular administration of heat-inactivated bacteria (Escherichia coli) is followed 2 h later by a drastic decrease in circulating hemocyte concentration, which is fully recovered only 4 days later. Apparently, bacteria are retained in the posterior kidney interstitium, where a perivascular accumulation of hemocytes is present, and where circulating hemocytes seem to converge after bacterial injection. Hemocyte proliferation (as indicated by the immunocytochemical detection of proliferative cell nuclear antigen, PCNA) occur in the renal interstitium after bacterial administration, and this proliferation is probably responsible for the recovery of the circulating hemocyte concentration that occurs later.