Pneumocystis carinii is an extracellular organism which is thought to require attachment to alveolar epithelial cells for its growth and replication in humans. Fibronectin (Fn) binding to P. carinii is essential for optimal P. carinii attachment. This study demonstrates that gp120, a 110-120-kD membrane glycoprotein on P. carinii, mediates attachment of the organism to cultured lung cells and is the site of Fn binding to P. carinii. A 51Cr-labeled P. carinii binding assay was used to quantify attachment of the organism to the alveolar epithelial cell line A549. Addition of free gp120, purified from whole P. carinii organisms, caused a significant decrease in attachment of P. carinii to A549 cells from 44.2 +/- 5.5% to 22.4 +/- 4.2% (P less than 0.01). Preincubation of the P. carinii organisms with a polyclonal antibody to gp120 also resulted in a marked decrease in P. carinii attachment to A549 cells from 46.8% +/- 5.2% to 21.3 +/- 4.8% (P less than 0.01). Furthermore, addition of free gp120 to P. carinii organisms caused a significant reduction in specific binding of 125I-Fn to P. carinii (from 83.3 +/- 8.5 ng to 47.1 +/- 5.9 ng, P less than 0.01). Similarly, anti-gp 120 antibody decreased specific Fn binding to P. carinii from 74.3 +/- 8.4 ng to 25.5 +/- 5.3 ng (P less than 0.001). Solubilized P. carinii organisms separated by gel electrophoresis and blotted with 125I-Fn demonstrated specific binding of the 125I-Fn to gp120. In addition, a specific anti-beta 1-integrin antiserum reacted with gp120 by Western blot, suggesting structural homology between gp120 and the beta-subunit of integrins. Thus, the data suggest that the P. carinii membrane glycoprotein gp120 functions as a Fn binding protein and is required for optimal P. carinii attachment to alveolar epithelial cells.