The stability of Lolium perenne extract was studied during a year in different conditions of storage. The changes of allergen activity were measured periodically by both in vivo and in vitro techniques, and structural modifications were examined by polyacrylamide gel electrophoresis and isoelectric focusing. Lyophilised extracts did not show any change and frozen samples retained full activity, but there were slight alterations in the pattern of proteins during the storage period. The activity of refrigerated aqueous extracts (4-6 degrees C) decreased gradually with time, while glycerinated samples at the same temperatures did not lose any of their allergen potency. Room temperature and 40 degrees C were unsatisfactory for aqueous extracts, but less so for preparations that contained 50% glycerol. The presence of the preservative phenol had a significant negative effect on stability at all of the temperatures studied.