Pneumocystis carinii obtained from rat lungs (RLH) and in vitro culture (RTC) were reacted with a panel of 14 fluorescein isothiocyanate conjugated lectins. Percentage fluorescence and fluorescent intensity were determined for both trophic and cyst forms. All RLH and RTC derived organisms bound strongly concanavalin A (Con A), and wheat germ agglutinin (WGA). However, differences in soybean agglutinin (SBA) binding between RLH and RTC organisms was observed. Different subsets of the organism bound lectins from Griffonia simplicifolia I, Maclura pomifera, and Bauhinia purpurea, indicating heterogeneity in the surface carbohydrates within each of the RLH and RTC populations. Eight lectins reacted very weakly or not at all: Dolichos biflorus, Arachis hypogaea, Griffonia simplicifolia I-beta 4, Solanum tuberosum, Ulex europeus, Griffonia simplicifolia II, Viscum album, and Limax flavus. The results indicate that P. carinii trophic and cyst forms have surface constituents containing mannose, N-acetylglucosamine and N-acetylgalactosamine as the predominant carbohydrates. Molecules resembling sialic acid and beta-galactose are absent or inaccessible. The surface glycoconjugates identified in these studies may play a role in the adherent properties of P. carinii.