Profilin, a ubiquitous low molecular weight (13,000-15,000 M(r)) actin binding protein, regulates the formation of F-actin structures in vivo, and is localized to specific cellular regions through interaction with proline-rich sequences. Here we report the 2.2 A X-ray structure of the complex between human platelet profilin (HPP) and a decamer of L-proline (L-Pro10). The L-Pro10 peptide adopts a left-handed type II poly-L-proline helix (PPII) and binds to a highly conserved patch of aromatic amino acids on the surface of profilin. The peptide and actin binding sites reside on orthogonal surfaces, and L-Pro10 binding does not result in a conformational rearrangement of HPP. This structure suggests a mechanism for the localization of profilin and its actin-related activities to sites of actin filament assembly in vivo.