The standardization of allergen extracts is of primary relevance to the clinical efficacy. Biological standardization procedures are widely used in the commercial production of vaccines. We tested, in grass-allergic patients, the potency of three different grass extracts for sublingual immunotherapy by means of skin prick tests. Specific IgE against Phl p 1, 2, 4, 5, 7, 11 and 12 were also assayed. Allerslit and Sublivac were directly applied as skin test. Grazax, was prepared by dissolving two tablets in 2 mL saline. Thirty-three subjects (mean age 38.8) were studied. The skin response was significantly different among extracts, decreasing from Allerslit to Grazax (t test < 0.01), but all the extract produced a skin response greater than histamine. All the subjects had specific IgE to Phl p 1 and Phl p 4 but 24% did not have specific IgE to Phl p 5. In those subjects the skin response to the three extracts did not differ from that of Phl p 5-positive subjects. Our findings confirm that there is a variability in the biological potency among different extracts. In addition, the standardization of grass extracts based on Phlp 5 only, may be insufficient in some cases.