Rat peritoneal eosinophils were examined after intraperitoneal infusion either of a mixture of phosphate-buffered saline (PBS) and colloidal gold or of fetal calf serum. These cells characteristically contained vesiculotubular structures, cuplike structures, and small granules during centrifugation. The cup-shaped structures and elaborate labyrinths of vacuole-like spaces increased markedly after injection of the PBS-colloidal gold mixture, presumably as features of heightened microendocytic activity. The vesiculotubular structures increased greatly after infusion of fetal calf serum. A few cyrstalloid granules exhibited fine-structural changes after the PBS-colloidal gold injection, and more numerous crystalloid granules appeared altered after fetal calf serum. Infrequent small granules contained a lucent, crystal-like silhouette after the fetal calf serum injection. Eosinophils evidenced microendocytic uptake of gold spherules into coated vesicles, the cup-shaped structures, and the small granules, but not into the vesiculotubular structures or crystalloid granules after intraperitoneal infusion of the PBS-gold mixture. Strong unmasked acid phosphatase activity in small granules contrasted with the general lack of activity in normal-appearing crystalloid granules and moderate activity in apparently altered crystalloid granules, presumably reflecting active and latent forms of enzyme in the different granules.

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