The complete primary structure of the major allergen Phl p 6 was determined by cDNA cloning. A cDNA library of timothy grass pollen was screened by a Phl p 6 directed mouse antiserum. Six clones were obtained. From the cDNA data we deduced a protein sequence of 110 amino acids. While the size of the leader sequences varied considerably between the different clones, only one amino acid exchange affected the mature proteins. Sequence comparison of group VI and V allergens showed a high degree of homology in the N- and C-terminal regions. The structural data strongly indicate that group VI and V are independent groups of allergens, which have derived from a common precursor gene.