Using the hydroxylamine-osmium tetroxide (HOT) technique, we have identified a constitutional point mutation in the retinoblastoma susceptibility gene (RB1) which segregates with the expression of retinoblastoma in five affected family members. One member developed a second primary tumor, a small-cell lung carcinoma (SCLC), which metastasized to the liver. Analysis of liver tumour DNA revealed homozygosity for the constitutional mutation, a G----A transition at the fifth base of intron 21, resulting in the excision of exon 21 from the mRNA. This is the first demonstration of homozygotization of a constitutional RB mutation in a metastatic second primary tumour and underlines the usefulness of the HOT technique for identification of mutations of the RB1 gene.