The purpose of this study was to develop a quantifiable method to fix 6-n-propylthiouracil (PROP) onto filter paper disks and to test the validity of the method relative to the three-solution test, previously developed in this laboratory. Filter paper disks were impregnated with 50 mmol/l PROP or 1.0 mol/l NaCl then dried. The concentration of PROP per disk was determined to be 0.280 mg+/-2.2% (CV) by ethanol extraction. Subjects were studied in two groups (n=62 per group). All subjects were initially classified by the three-solution test and then were independently classified by the paper disk test. In the paper disk test, group means for PROP (+/-95% confidence interval) were calculated and used to establish numerical cutoff scores for taster group classification. Cutoff scores for Group 1 were used to classify subjects in Group 2, verifying that the cutoffs were applicable to another subject group. The percentage of nontasters, medium tasters, and supertasters identified by paper disk was 27%, 42%, and 31%, respectively, which is consistent with expected frequencies in the population. For Group 1, the agreement between the two classification methods ranged from 86% to 94% across taster groups. For Group 2, the agreement ranged from 83% to 100%. The contingency coefficient (P) of the degree of association between the two classification methods was high (P=0.77 and P=0.74 for Groups 1 and 2, respectively; P< or =.001). These results demonstrate that the paper disk method is a reliable screening tool for assessing sensitivity to PROP that has numerous applications in basic and applied research.