A new class of covalently linkable platinum cluster reagents with core diameters close to 2 nm has been prepared. The new label offers the performance advantages and versatility of the 1.4-nm Nanogold in a larger label which is more clearly visualized against electron-dense regions of a specimen. Large platinum clusters were prepared by the reduction of platinum(II) acetate in the presence of 1,10-phenanthroline ligands which had been synthetically modified to include solubilizing and reactive cross-linkable functional groups. These were then conjugated site-specifically to antibody IgG molecules and to Fab' fragments and visualized by scanning transmission electron microscopy. The resulting conjugates may be autometallographically enhanced and show sensitivity similar to that of Nanogold conjugates in immunoblotting experiments. In preliminary experiments, they have also exhibited labeling of tissue antigens and penetrate to access nuclear targets.