Three known allergens of yellow jacket (Vespula vulgaris) venom are antigen 5, hyaluronidase, and phospholipase. Yellow jacket antigen 5 has been previously cloned and expressed in bacteria; it contains 204 amino acid residues, and it has 69% and 60% sequence identities with the homologous proteins of white-faced hornet (Dolichovespula maculata) and wasp (Polistes annularis), respectively. These studies are now extended to yellow jacket hyaluronidase and phospholipase; they contain 331 and 300 amino acid residues, respectively, and they show 92% and 67% sequence identity with their homologs of white-faced hornet. Tests with the natural and the recombinant vespid allergens in mice indicate partial antigenic cross-reactivity of their homologous proteins at both B- and T-cell levels. There is greater cross-reactivity among hornet and yellow jacket allergens than that among hornet or yellow jacket and wasp allergens. The order of cross-reaction of the three vespid allergens is hyaluronidase > antigen 5 > phospholipase. The continuous (linear) B-cell epitopes of vespid allergens show greater cross-reactivity than their discontinuous epitopes do. The discontinuous B-cell epitopes are immunodominant for all vespid allergens. The low degree of cross-reactivity of the immunodominant discontinuous B-cell epitopes of vespid allergens should be taken into consideration in selection of venoms for immunotherapy of patients with sensitivity to multiple vespids.