The concentration of allergen-specific IgE-antibodies in the sera from 47 patients with hypersensitivity to horse dandruff, birch or timothy pollen was estimated with the radioallergosorbent test (RAST). The biological effect of the reagins in these sera was studied with the chopped human lung test (CHLT) regarding their ability to sensitize lung tissue passively. A highly significant correlation was obtained between the results of RAST and the serum end dilutions in CHLT for all allergens used. Because of this and the great capacity of RAST this method seems to be very suitable for studies on reagin-mediated allergies. In both in vitro methods used, an interference of other serum constituents sometimes resulted in falsely low values but this error seemed to be overcome by testing diluted sera.