Gold nanoparticles having a particle size of 4-15 nM are synthesized by the reduction of hydrogen tetrachloroaurate (III) trihydrate using a suitable reducing agent such as sodium citrate in the presence of unmodified .alpha., .beta. or .gamma.-cyclodextrin. The particle size is dependent upon the type and concentration of cyclodextrin and the reducing agent concentration. Gold nanoparticles having a particle size of 2-4 nM are produced by the reduction of hydrogen tetrachloroaurate (III) trihydrate using sodium borohydride in the presence of cyclodextrins.