The first part of the present paper was published recently (DE BELLEFEUILLE 1961) in this journal. Existing levels~of radiationIn the estimation of genetic hazards, attempts are made to establish the amount, spread over the whole population, received by the human testis or ovary from conception to the age of 30 years, by which time one-half of all offspring have been procreated, Natural radiation from the ground, from cosmic rays and from potassium 40 in human tissues represents a 30-year gonad dose of the order of 3 r (MRC Report) to 4.3 r (see: 'Genetic Effects of Atomic Radiation'). It is in the light of this estimate that one may appraise the significance of man-made radiation impinging upon human germ plasm.Fall-out from test detonations of nuclear weapons consists of a large number of radio-nuclides liberated into the atmosphere . These present a two-fold genetic danger. External radiation comes to the gonads from the ground; internal radiation is due to isotopes which are incorporated into human tissues. Of these, the bone-seekers (strontium 89 and 90, plutonium 239) emit short-range p-particles; they are thus much less significant in terms of genetic effects than in relation to the induction of leukaemia and bone