We have shown previously that mast cells (MC) and MC-derived mediators are crucial for early stages of zymosan-induced peritonitis in Swiss and Balb/c mice. Present results revealed the same phenomena in males of C57Bl/6 mice as in the MC-depleted animals (by application of a compound 48/80) the intraperitoneal accumulation of exudatory fluid and cells, measured at 0.5, 6 and 24 hours after zymosan injection, was significantly diminished. In particular, an early vascular permeability and PGE2 content at 30 min of peritonitis, and exudatory leukocyte numbers and MCP-1 content at 6 hours of peritonitis were significantly lower in MC-deprived mice then in those with normal MC population. The peritoneal cavity of intact C57Bl/6 males contains less then 1% of mast cells defined as cells filled with metachromatically stained granules (counted in a haemocytometer) and those with MC-specific c-kit receptors (determined both by quantitative PCR at mRNA level and by flow cytometry on cell surface). After zymosan injection numbers of cells with metachromatically-stained granules significantly decrease reaching minimum at 6 hours of peritonitis with a parallel significant increase of c-kit positive MCs. These contradictory results might be explained by zymosan-induced degranulation of mast cells and concomitant influx of newly arriving mast cells through the omental milky spots and/or in situ MC proliferation.