We have used a non-isotopic PCR assay based on the chemiluminescent detection of blotted PCR products (CB-PCR) for two dynamic mutation diseases (Huntington's disease and spinocerebellar ataxia type 1). This gives an accurate sizing of alleles and permits a rapid analysis of at risk persons. The system involves PCR of the samples, separation of alleles on polyacrylamide gels, Southern blotting, and hybridisation with specific primers 3' labelled with fluorescein (F1)-dUTP as probes. CB-PCR retains the isotopic sensitivity for accurate allele determination, avoids isotopic manipulation, and provides the advantages of safety, long term storage of probes, and recycling of hybridisation solutions.