Mouthwashes can be used as a DNA resource for mutation detection and, because collection and DNA isolation is simple and cheap, they could in particular, be used for large numbers of samples. To determine the failure rate (the proportion of mouth samples in which no PCR product was obtained) and the specificity of buccal epithelial cell mutation detection in large numbers of samples, we collected mouthwashes and blood samples from 11 413 blood donors and tested the mouthwashes for the deltaF508 mutation, which has an estimated frequency Of 75% among cystic fibrosis chromosomes in The Netherlands. Blood samples were tested for the deltaF508 mutations only if the mutation was identified in the mouthwash or in the case of a failure to obtain PCR products. The sensitivity of the test was determined in mouthwashes of 75 deltaF508 carriers known from earlier family studies. These samples were offered blindly between the mouthwashes of the blood donors. Both specificity and sensitivity of the mouthwash procedure were 100%. The overall failure rate was 5.6%. This large figure was caused mainly by insufficient rinsing of the mouth in one particular blood bank. Exclusion of the results of this blood bank reduced the failure rate to 1.8%. Our results also confirmed that for a large number of samples the mouthwash procedure is suitable for mutation detection and, with proper instructions, can be used in community screening.