Nanoscale objects are typically internalized by cells into membrane-bounded endosomes and fail to access the cytosolic cell machinery. Whereas some biomacromols. may penetrate or fuse with cell membranes without overt membrane disruption, no synthetic material of comparable size has shown this property yet. Cationic nano-objects pass through cell membranes by generating transient holes, a process assocd. with cytotoxicity. Studies aimed at generating cell-penetrating nanomaterials have focused on the effect of size, shape and compn. Here, we compare membrane penetration by two nanoparticle isomers' with similar compn. (same hydrophobic content), one coated with subnanometre striations of alternating anionic and hydrophobic groups, and the other coated with the same moieties but in a random distribution. We show that the former particles penetrate the plasma membrane without bilayer disruption, whereas the latter are mostly trapped in endosomes. Our results offer a paradigm for analyzing the fundamental problem of cell-membrane-penetrating bio- and macro-mols. The structural organization of surface groups on nanoparticles is proven to be important for cell membrane penetration. Nanoparticles coated with alternating ribbon-like arrangements of hydrophobic and anionic ligands penetrate membranes without causing disruption. These design rules may have implications for toxicity issues and drug delivery applications of nanomaterials. The structural organization of surface groups on nanoparticles is proven to be important for cell membrane penetration. Nanoparticles coated with alternating ribbon-like arrangements of hydrophobic and anionic ligands penetrate membranes without causing disruption. These design rules may have implications for toxicity issues and drug delivery applications of nanomaterials.