The neutrophil and macrophage responses that accompany inflammation were studied in the peritoneal cavity of rainbow trout Oncorhynchus mykiss using light and electron microscopic cytochemistry. Neutrophils of inflamnlatory pentoneal exudates were alpha-naphthyl butyrate esterasenegative, peroxldase-positive and rich in cytoplasmic glycogen granules. Macrophages were poor in glycogen, esterase-positive and usually peroxidase-negative. Some peroxidase-posibve macrophages were due to the transfer to macrophages of neutrophilic peroxidase. The ultrastructural double labehng for glycogen/peroxidase or esterase/peroxidase was most useful for precisely characterising neutrophils and macrophages in the inflamed peritoneal cavities and for correctly labelling peroxidasepositive macrophages. Intraperitoneal injection of casein, Incomplete Freund's Adjuvant (IFA) and live or formol-killed Yersinia ruckeri resulted in a rapid influx of neutrophils, peaking at 24 to 48 h postinjection and reaching values, in the case of live bacteria, 5OOx those in the resting, unstimulated peritoneal cavity. Peritonea1 macrophages also increased, but the response was slower (peak at 5 d ) and with more modest increases in number ( 7 . 5~) Neutrophil and mononuclear cells returned to normal values after 15 d in the case of casein and bacteria, but continued above base values 30 d after the injection of IFA. Conversely, after the injection of phosphate buffered s a h e , India ink or with shaminjections, very moderate neutrophil and macrophage responses subsided in a few hours. Phagocytosis of bacteria was studied by light microscopy of preparations stained for peroxidase by a new method which allows for the simultaneous observation of intracellular bacteria and peroxidase staining. When Y. ruckeri was injected into resting peritoneal cavities, bacteria were ingested by the resident macrophages. When the bacteria were injected into cavities with high numbers of neutrophils (due to the previous injection of casein), more neutrophils than macrophages contained bacteria. Results show that the macrophages are the resident phagocytes of the peritoneal cavity of trout, while neutrophils are present in that body cavity in significant numbers only in situations of inflammation and only as long as the inflammation persists.

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Collagen deposition within granulomas formed after Mycobacterium avium infection was analysed on histological sections stained with Masson's trichrome using acquired computerized image analysis and a program that was specifically designed for that purpose. Comparison was made between immunocompetent C57BL/6 mice and mice genetically deficient in the inducible nitric oxide (NO) synthase gene (iNOS(-/-) mice) infected with either a highly virulent strain or a moderately virulent strain of M. avium. iNOS-deficient mice were more resistant to the highly virulent strain than control C57B1/6 mice, but both strains were equally susceptible to the less virulent M. avium strain. Collagen distribution in the granuloma was found in the cuff surrounding the granuloma in an area rich in lymphoid cells as well as inside the granuloma itself, conferring a mesh-like structure within that lesion. It was seen that iNOS(-/-) mice induced a higher collagen deposition than C57BL/6 mice and that such collagen deposition varied with the mycobacterial strain used to infect the animals.

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