The spongiform encephalopathy Creutzfeldt-Jakob disease (CJD) has been transmitted to man via administration of growth hormone and gonadotropin extracted from large pooled batches of human cadaveric pituitary glands. In the UK, 1908 individuals were exposed to potentially contaminated growth hormone, of whom 6 have so far manifested CJD. Examination of the prion protein genes of all these cases and of a single case of gonadotropin-related CJD showed that 4 had the uncommon valine 129 homozygous genotype indicating genetic susceptibility to prion infection. Such genetic susceptibility may be important in the aetiology of sporadic CJD disease.