Pneumocystis carinii are coated by abundant surface proteins of great interest to investigators [1–24]. The major protein components of both cysts and trophozoites are proteins called MSG (major surface glycoprotein; see [23–25]), P115 [9], gp120 [11] or gpA [19, 20], with an apparent molecular mass ranging from 95 to 140 kDa, depending on host species, degree of glycosylation, and isolation procedures. The nomenclature for this antigen complex is somewhat confusing, and we refer to it here as MSG as it represents closely the nature of this antigen. Upon P. carinii infection, the organism passed through the initial host defenses is deposited within alveoli. The initial event in the pathogenesis of P. carinii pneumonia is the preferential attachment of the organism to type I alveolar epithelial cells (for review see [26]). Although the mechanisms of attachment are not fully understood, MSG is thought to play an important role in the interaction of P. carinii with host cells, involving other components such as cell adhesive proteins and lectins [7, 27–35]. P. carinii trophozoites can bind to alveolar epithelial cells by using fibronectin as a ligand, and MSG has been implicated as the binding site for fibronectin. Other extracellular matrix adhesive proteins that may be involved include laminin and vitronectin. P. carinii may also bind to epithelial cell surface glycoproteins by means of a mannose-dependent mechanism [36]. Thus, attachment to type-I alveolar epithelial cells may be mediated by multiple mechanisms. There is evidence that P. carinii infection stimulates humoral and cellular immune responses in the host. For example, passive immunization with an anti-MSG monoclonal antibody partially protects against the progression of P. carinii pneumonia in animal models [37], and a specific T-cell response to MSG molecules occurs after immunization …