Ag(CN)(2)(-) is reduced at a low rate by radiolytically generated hydroxymethyl radicals. The reduction occurs on tiny nuclei in solution formed by hydrolysis. When colloidal silver seed particles are present in the solution, the reduction of Ag(CN)(2)(-) is much faster, and larger silver particles with a narrow size distribution are produced. A mechanism is proposed, in which the . CH2OH radicals transfer electrons to the seed particles, and the stored electrons finally reduce Ag(CN)(2)(-) directly on the surface of the seeds. The limitations of this kind of radiolytic particle enlargement are discussed. In the presence of colloidal platinum, Ag(CN)(2)(-) is also rapidly reduced by the organic radicals. Bimetallic particles of the PtcoreAgshell type with a rather nonsymmetric shape of the shell are formed; despite this irregular structure, the optical spectra agree fairly well with literature spectra calculated for symmetric bimetallic particles.