Flow cytometric analysis of FR⁺ peritoneal cells. Representative flow cytometric analyses of FR expression on (A) thioglycolate-recruited, (B) bacteria-recruited, and (C) quiescent peritoneal cells. Three days after intraperitoneal injection of thioglycolate, live P aeruginosa, or sterile PBS, peritoneal cells were removed and analyzed by flow cytometry. On the basis of side scatter and F4/80 fluorescence, 3 regions (R1-R3) of density plots were defined. Cells in R1 are macrophages; cells in R2 granulocytes; cells in R3 lymphocytes and a few erythrocytes. The cell suspension was stained with 100 nM folate-FITC in the absence (solid black histogram) or presence of an excess (10 μM) of free folic acid to competitively occupy FR (filled gray histogram). The percentage of FR⁺ cells (average from at least 3 independent experiments) within each gate is shown.