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The interaction of Legionella micdadei with human polymorphonuclear neutrophils (PMNs) and serum was investigated to determine their roles in host defense against this organism. Serum and PMNs from normal donors having no antibody to L micadei were used. PMNs phagocytized once-agar-passaged (74.6% +/- 6.5%) and twice-agar-passaged (87.3% +/- 1.0%) L micdadei less (P less than 0.05) than L micdadei passaged multiple times on agar (97.5% +/- 1.0%) or Staphylococcus aureus (98.3% +/- 0.5%). Under the same conditions, no phagocytosis of Legionella pneumophila occurred. Use of heat-inactivated serum abolished phagocytosis. PMN killing of once-agar-passaged (1.5% +/- 1.5%) and twice-agar-passaged (0) L micdadei was less (P less than 0.001) than that of L micdadei passaged multiple times on agar (88.6% +/- 4.0%) or S aureus (96.8% +/- 0.5%). L micdadei was not killed by fresh serum, although, in contrast to L pneumophila, it was opsonized by C3. Thus virulent L micdadei is phagocytized, but not killed, by human PMNs, with complement being the major opsonin.