The FAST assay (Allergenetics) for the determination of specific IgE has recently been introduced. The results of this test, graded in classes 0 (negative), 1 (equivocal) and 2 to 6 (positive) were compared with those of another commercially available test (Phadezym RAST, Pharmacia), graded from class 0 (negative) and classes 1 to 4 (positive). In 52 adults suffering from rhinitis/asthma, a total of 114 positive skin prick tests (SPT) were obtained for common respiratory allergens. In 67% of these tests the Phadezym RAST was positive and in 63% the FAST was positive (classes 2 to 6). In these patients there were 151 negative SPT: 6% corresponded to a positive Phadezym RAST (all class 1) and 34% to a positive FAST (classes 2 to 6). The serum of nine nonatopic volunteers who had negative SPT for 12 common allergens were tested. In none did the Phadezym RAST give any positive results; the FAST was positive in all nine sera, detecting between five and 11 allergens. When both assays were performed on 14 unselected cord blood samples, the Phadezym RAST was positive in three samples (with class 1 results to a maximum of two allergens), and the FAST was positive in 12 samples, detecting between one and seven allergens. Thus, in our hands, the FAST gave an abnormally high number of positive results in patients with negative SPT, in nonatopic volunteers, and in cord blood.