With the recent identification of the cystic fibrosis gene and the elucidation of one of the major mutations responsible for the disease, it is now possible to screen directly for carriers of this particular mutation at the DNA level. The mutation that accounts for approximately 70% of the affected population in North America has been identified as a deletion of a phenylalanine residue at position 508 of the deduced amino acid sequence of the putative protein. We describe herein a method for screening carriers of this mutation which makes use of the polymerase chain reaction and direct viewing of the mutation by staining with ethidium bromide after electrophoresis of the polymerase chain reaction products on a polyacrylamide gel. This quick and sensitive method avoids the need for hybridization assays and the hazard associated with isotopically labeled probes.