The binding characteristics of a panel of commercially available FITC-conjugated lectins to Pneumocystis carinii (Pc) were assessed by fluorescence microscopy and flow cytometry. Rat Pc obtained from infected lung homogenates were incubated with FITC-conjugated lectins in a series of concentrations, counterstained with propidium iodide, and analyzed for percent fluorescence and fluorescence intensity. All organisms bound concanavalin A and Wisteria floribunda agglutinin, 2 representatives of the glucose/mannose-binding group. From the lectin group specific for N-acetylglucosamine, Pc reacted more strongly with wheat germ agglutinin than with Solanum tuberosum agglutinin or Griffonia simplicifolia II lectin. Pneumocystis treated with lectins specific for N-acetyl-D-galactosamine and galactose exhibited much variation; the cells reacted moderately well to soybean agglutinin and less to Bauhinia purpurea, Maclura pomifera and Dolichos biflorus agglutinins and Griffonia simplicifolia I lectin. Arachis hypogaea agglutinin, Viscum album agglutinin and Griffonia simplicifolia I-beta 4 lectin had not effect. The organisms reacted weakly with Ulex europeus I agglutinin which is specific for fucose and did not react with Limax flavus lectin, which is specific for sialic acid. Competitive inhibition studies using relevant carbohydrates were performed to indicate that the positive reactions were specific. These studies should help to elucidate the mechanisms of attachment and pathogenesis of this organism.