A purified freeze-dried preparation of timothy pollen was compared with a crude extract in RAST, skin tests, conjunctival provocation tests and desensitization treatment. In 48 children with grass pollen allergy a thorough allergological investigation was performed, using the two allergen preparations in different tests. The purified preparation contained relevant allergens, as judged from the good agreement between the two extracts in prick and provocation tests. The freeze-dried purified extract had good storage stability. Desensitization treatment was found to have a significant effect, as judged by repeated prick and provocation tests with both extracts. The effect of the desensitization treatment was at least as good in the children treated with the purified preparation as in those treated with the crude extract.