Complement requirement for phagocytosis. Conditions for phagocytosis were as described in Methods and the legend for Table 2. After 7 minutes of tumbling at 37°C, bacteria-neutrophil suspensions were diluted in cold HHBSS and deposited on charged microscope slides by cytocentrifugation. Slides were stained with Safranin, and neutrophils were scored for association with bacteria as described in Methods. (a) Neutrophil interactions with E. coli opsonized by fresh autologous serum. (b) Neutrophil interactions with E. coli opsonized in parallel by complement-inactivated (30 minutes at 56°C) autologous serum. Bacteria are agglutinated but not ingested. Most neutrophils contain no bacteria. The neutrophil in the center of the field has a tangentially adherent (appliqué) bacterium. (c) Frequency histogram of neutrophil-bacteria associations, as described in Methods. Black bars: bacteria opsonized by native serum; white bars: bacteria opsonized by complement-inactivated serum. Error bars reflect the SEM of data from five experiments.