Data on the distribution of dicentrics and acentrics observed when human lymphocytes are cultured for 48 h after irradiation by X-rays, γ-rays, and neutrons are presented. Analysis shows that for dicentrics, the observed distribution for X-rays, γ-rays, and fission neutrons may be described by Poisson statistics but for higher energy neutrons overdispersion is observed. The phenomenon of overdispersion is also observed for acentrics irrespective of the radiation used. The possibility that overdispersion results from the variations of dose in sensitive sites leads to the conclusion that for dicentrics the site size is considerably larger than the 1-2 μm diameter derived by applying the dual action theory to the dose effect relationships. This larger site may well be the cell nucleus.