Four families with suspected hereditary retinoblastoma in proband and one other family member were examined by genetic segregation analysis of the RB-1 gene loci with specific RFLPs of chromosome 13. Two families, TA-6 and partially CK-46, were informative using this method. In TA-6 family healthy sister (TA-6/10) of the proband was shown not to be a carrier of the mutant RB-1 gene. These results show that the segregation analysis of RB-1 gene loci with specific RFLPs, used as the DNA markers, could be helpful in the genetic diagnosis and counselling in families with hereditary retinoblastoma.