By adjustment of solvent conditions for synthesis, virtually monodisperse 4-mercaptobenzoic acid (p-MBA) monolayer-protected gold nanoparticles, 2 and 3 nm in diameter, were obtained. Large single crystals of the 2 nm particles could be grown from the reaction mixture. Uniformity was also demonstrated by the formation of two-dimensional arrays and by quantitative high-angle annular dark-field scanning transmission electron microscopy. The 2 and 3 nm particles were spontaneously reactive for conjugation with proteins and DNA, and further reaction could be prevented by repassivation with glutathione. Conjugates with antibody Fc fragment could be used to identify TAP-tagged proteins of interest in electron micrographs, through the binding of a pair of particles to the pair of protein A domains in the TAP tag.