Serum opacity factor (SOF) is a unique multifunctional virulence determinant expressed at the surface of Streptococcus pyogenes and has been shown to elicit protective immunity against GAS infection in a murine challenge model. SOF consists of two distinct domains with different binding capacities: an N-terminal domain that binds apolipoprotein AI and a C-terminal repeat domain that binds fibronectin and fibrinogen. The capacity of SOF to opacify serum by disrupting the structure of high density lipoproteins may preclude its use as a vaccine antigen in humans. This study generated mutant forms of recombinant SOF with reduced (100-fold) or abrogated opacity factor (OF) activity, for use as vaccine antigens. However, alterations introduced into the N-terminal SOF peptide (SOFFn) by mutagenesis to abrogate OF activity, abolish the capacity of SOF to protect against lethal systemic S. pyogenes challenge in a murine model. Mutant forms of purified SOFFn peptide were also used to assess the contribution of OF activity to the pathogenic processes of cell adhesion and cell invasion. Using latex beads coated with full-length SOF, SOFFn peptide, or a peptide encompassing the C-terminal repeats (FnBD), we demonstrate that adhesion to HEp-2 cells is mediated by both SOFFn and FnBD. The HEp-2 cell binding displayed by the N-terminal SOFFn peptide is independent of OF activity. We demonstrate that while the N terminus of SOF does not directly mediate intracellular uptake by epithelial cells, this domain enhances epithelial cell uptake mediated by full-length SOF, in comparison to the FnBD alone.