In order to determine the possible mechanisms whereby interactions between phagocytic cells and crystals of monosodium urate (MSU) lead to cell death with simultaneous release of both cytoplasmic and lysosomal enzymes, phagocytic leukocytes of the smooth dogfish shark Mustelus canis were studied by means of light and electron microscopy, and biochemistry. Lysosomes of these cells can be stained supravitally with toluidine blue and are large enough (0.7–0.8 m̈) to be clearly resolved with the light microscope. Light microscopic observations showed that of cells exposed to MSU 87% of those containing visible ingested crystals died within 1 hour, whereas 92% of adjacent cells in the same wet mount without such crystals survived. Cell death occured after a latent period of 10–15 minutes following fusion of lysosomes with crystal-containing phagosomes. Electron microscopic examination of both dogfish and human leukocytes exposed to MSU for more than 1 hour and then fixed in situ revealed occasional discontinuities or ruptures in secondary lysosome membranes. Endogenous peroxidase activity could be cytochemically localized in primary and secondary lysosomes and in the cytoplasm adjacent to such ruptured secondary lysosomes. It was not seen adjacent to primary lysosomes, a result indicating that the cytoplasmic reaction product was not a diffusion artifact. To exclude the possibility that crystals were exercising their affect primarily upon the plasma membrane, suspensions of dogfish buffy coat cells were incubated with cytochalasin B (5 m̈g/ml, 10 minutes), which inhibits phagocytosis but not exocytosis of lysosomal enzymes by stimulated phagocytes. Whereas cells exposed to MSU crystals released 30% of their content of lysosomal β-glucuronidase activity and 28% of their cytoplasmic lactate dehydrogenase (LDH) activity within 3 hours, preincubation with cytochalasin B reduced the release of LDH activity within that period to 6% but reduced the release of β-glucuronidase activity only to 20%. Preincubation with 10–3M cyclic adenosine monophosphate (cAMP) and theophylline (10–3M), which inhibit lysosomal fusion, reduced the release of both LDH and β-glucuronidase activities to 7% and 6% respectively. Cells that were preincubated with both cytochalasin B and cAMP + theophylline released only 1% LDH activity and 4% β-glucuronidase activity. These results are compatible with the “suicide sac” hypothesis of lysosomal enzyme release mediated by MSU for the following reasons: a) cell death was seen to follow uptake, not mere exposure to crystals, b) ultrastructural studies indicated that the primary injury was to the secondary lysosome membrane, and c) cell death was reduced when either phagocytosis or lysosomal fusion was inhibited.