Two isoallergens of Cyn d I were isolated using preparative isoelectric focussing, and were designated Cyn d Ia and b. These isoallergens differ in their pI, molecular weight (Cyn d Ia, 32 kD and Cyn d Ib, 31 kD) and their NH2-terminal sequence. Four monoclonal antibodies (Mabs) were produced using Cyn d Ia as antigen. These Mabs recognized both Cyn d Ia and b. One of the Mabs recognized four different pI forms of Cyn d I on 2D gels. The Mabs also recognized cross-reactive epitopes on proteins from eight other grasses including rye grass, timothy grass, Kentucky bluegrass and Johnson grass.