Certain strains of Streptococcus sanguis adhere selectively to human platelets (Adh+) and, in plasma, induce them to aggregate into in vitro thrombi (Agg+). The induction of aggregation is mediated by the platelet aggregation-associated protein (PAAP) expressed on the cell surface of the streptococcus. In endocarditis, expression of PAAP may be regulated by association with host proteins on damaged heart valves. To begin to test this hypothesis, three strains of S. sanguis were each cultured in the presence or absence of collagens (types I to X), laminin, or PAAP-derived peptide preparations. After harvesting and washing, the platelet-interactive phenotype of strains 133-79 (Adh+ Agg+), L74 (Adh+ Agg-), and 10556 (Adh- Agg-) was unchanged. The cells from each culture were then digested mildly with trypsin to isolate PAAP. PAAP isolated from strain 133-79 (Adh+ Agg+) grown in the absence of added collagen, other proteins, or peptides inhibited platelet aggregation in response to untreated cells of S. sanguis. Platelet aggregation was induced immediately, however, by PAAP from strain 133-79 isolated after growth in the presence of 300 nM type I collagen, while lower concentrations yielded protein fragments that potentiated the response to intact cells. Aggregation-inducing PAAP could be removed by anti-PAAP (PGEQGPK) immunoaffinity chromatography, but only inhibitory activity could be recovered. The agonist effect of PAAP was not associated with collagen itself, since the PAAP preparations did not contain detectable amounts of hydroxyproline. PAAP antigens isolated from cells grown in the presence and absence of collagen had similar apparent molecular weights, as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western immunoblotting. When electrophoresis was performed under nondenaturing conditions, however, PAAP isolated from cells grown in type I collagen migrated more slowly. Strain L74 grown with type I collagen yielded tryptic fragments of proteins that inhibited aggregation significantly better than control peptides (no collagen in the medium). Strain 10556 was apparently unaffected by growth in type I collagen. The effect of type I collagen was somewhat unique. Growth in the presence of collagen types II to VI (300 nM) yielded protein fragments that potentiated without inducing platelet aggregation, while other collagens, laminin, and PAAP-derived peptides did not affect platelet aggregation. These results suggest that growth in the presence of type I collagen and, perhaps, collagens II to VI alters the expression and conformation of PAAP in certain strains of S. sanguis.