We purified from natural rubber latex (NRL) by means of high-performance liquid chromatography a 27-kD protein, recognized characteristically by IgE in sera from latex-allergic children with spina bifida or other congenital anomalies and histories of multiple surgeries. N-terminal sequence analysis of the purified 27-kD protein was unsuccessful suggesting that its N-terminus is blocked. To obtain internal sequence information from the protein it was digested with trypsin and the purified tryptic peptides were subjected to sequence analysis. Thirteen of the 14 sequenced peptides revealed no significant homology to any of the published protein sequences indicating that the 27-kD protein is previously undescribed at the primary structure level. However, one of the 14 sequenced peptides showed significant homology to the rubber elongation factor, a 14.6-kD NRL protein. For the time being, the 27-kD NRL protein is the first molecularly characterized NRL allergen associated with defined clinical manifestations of latex allergy.