Previous observations have demonstrated that monoclonal and polyclonal antibodies directed at SAG-1, the major surface protein of Toxoplasma gondii, decreased the number of T. gondii that infected fibroblast monolayers. Direct evaluation of parasite-host cell attachment using glutaraldehyde-fixed human fibroblasts and live tachyzoites was performed to determine whether SAG-1 was a ligand for the host cell receptor. The interaction between the fixed cells and T. gondii was specific and saturable as determined by a radioisotope competitive binding assay. Moreover, the specificity of this interaction was confirmed by comparison to another member of the Apicomplexa, Besnoitia jellisoni. Treatment of fresh extracellular T. gondii with rabbit polyclonal anti-SAG-1 serum inhibited parasite attachment to host cells by 71%. A monoclonal antibody (6A8) directed at SAG-1 was able to inhibit parasite binding to fixed host cells by 65%. Other mAb's directed at SAG-1 failed to inhibit parasite attachment in this assay. Fab derived from 6A8 mAb showed dose-dependent inhibition of parasite attachment. At an Fab concentration of 25 micrograms/ml, 47% inhibition was observed. Attachment assays using mutant parasites with defective SAG-1 (PTgA and PTgC) showed significantly reduced binding (26 and 39%) when compared to wild-type (SAG-1+) parentals. Taken together, these observations suggest that SAG-1 is an important parasite ligand that binds to the host cell in the process of T. gondii invasion.