To test whether lysosomal degranulation of phagocytes is associated with antibody-dependent cytotoxicity, eggs of Arbacia punctulata were used as targets for blood phagocytes of Mustelus canis. Eggs were coated with heat-aggregated dogfish IgM and exposed to phagocytes, and cytolysis of eggs was observed by Nomarski optics. Phagocytes adhered, degranulated, and raised fertilization membranes resembling those induced by sperm or ionophore A23187. Lysis was then observed as damage radiating from the point of phagocyte-egg contact. By 4 hr, coated eggs exposed to phagocytes released 8.9, 12.3, and 7.4% of total catalase (EC 220.127.116.11), beta-glucuronidase (EC 18.104.22.168), and superoxide dismutase (EC 22.214.171.124) into the medium. Cytotoxic enzyme release significantly exceeded that from uncoated eggs incubated with phagocytes or eggs alone (uncoated or coated). Because activated eggs release a neutral protease, it was considered possible that this enzyme might be responsible for autolysis of eggs. This possibility was excluded because (i) lysis of eggs was not inhibited by soybean trypsin inhibitor (SBTI) whereas the egg protease was sensitive to SBTI, and (ii) the major trypsin-like activity of phagocytes was not inhibited by SBTI. These experiments demonstrate that Ig-coated cells are first activated, and then killed, when exposed to degranulating phagocytes and suggest that enzymes from attacking phagocytes, and not target cells, are responsible for cell death.