The present invention provides isolated peptides of Lol p V, a major protein allergen of the species Lolium perenne. Therapeutic peptides within the scope of the invention comprise at least one T cell epitope, or preferably at least two T cell epitopes of a protein allergen of Lol p V. Diagnostic peptides within the scope of the invention bind IgE. The invention also provides modified peptides having similar or enhanced therapeutic properties or other desirable properties as the corresponding, naturally-occurring allergen or portion thereof. The invention further provides nucleic acid sequences coding for peptides of the invention. Use of the therapeutic compositions comprising one or more peptides of the invention in the manufacture of medicaments for treating sensitivity to Lol p V or an allergen immunologically related to Lol p V, or for general ryegrass sensitivity in an individual, is also provided. The invention also provides nucleic acid sequence coding for Dac g V protein allergen as well as the amino acid sequence of Dac g V protein allergen.