Pneumocystis carinii pneumonia is a major opportunistic infection in patients with the acquired immune deficiency syndrome. P. carinii attachment to alveolar epithelial cells is considered necessary for growth and replication of the organism. Recent studies have focused on the role of adhesive proteins such as fibronectin and vitronectin in attachment mechanisms of P. carinii in the alveolar space. Whereas the role of fibronectin has been partially characterized, less is known about the mechanism of vitronectin interaction with P. carinii. To better understand the mechanism underlying this interaction, vitronectin-P. carinii binding was characterized with respect to monovalent and divalent cations and pH by using an iodine 125-labeled vitronectin binding assay to P. carinii. As an example, vitronectin-P. carinii binding was abolished in the presence of 1.0 mol/L NaCl and enhanced by Ca2+ and Mn2+. Further, periodate and heparin treatment of P. carinii significantly reduced vitronectin binding to the organism to 10% +/- 1.5% (p < 0.01) and 52% +/- 1.8% (p < 0.05) of control values, respectively. There was no competitive inhibition of vitronectin binding to P. carinii by using the peptide sequence arg-gly-asp-ser of the cell binding domain. The findings suggest that vitronectin, unlike fibronectin, binds to P. carinii by a predominantly electrostatic mechanism that likely involves the heparin binding domain of vitronectin.