Pollen extracts are stable in solutions containing 50% glycerol. The stability of Dermatophagoides pteronyssinus (Dp) extracts have not been determined. Two lyophilised extracts (Dp and cocksfoot pollen) were reconstituted in 50% glycerol and stored for up to 6 months at +4 degrees C. Every day the extracts were placed at room temperature for 1 hr to mimic the natural use of allergen extracts by allergists. Every 2 months the extracts initially prepared were compared to freshly reconstituted extracts by means of prick skin tests in fifteen allergic patients. It was observed that cocksfoot pollen extract is stable but there appears a slight but significant (P less than 0.05) decay in activity when the extract stored for up to 6 months was compared with a freshly prepared extract. On the other hand there is a significant (P less than 0.01) decay in activity of the Dp extract after 2 months of storage. Dermatophagoides pteronyssinus and cocksfoot pollen extracts behave differently when stored. These findings have particular importance in the diagnosis and treatment of allergic disorders.