Perennial hyposensitization with a partially purified timothy extract resulted in a statistically significantly higher degree of clinical protection than treatment with the two timothy major allergens (Nos. 19 and 25) and protected better from the second--than during the first grass pollen season. The extracts were standardized biologically and adsorbed to aluminium hydroxide for administration. The therapy had a more beneficial influence on sneezing than on rhinorrhoea and blockage of nasal airways, and an excellent effect on grass pollen asthma was obtained with the partially purified timothy extract. Associated birch pollen allergy was not influenced by hyposensitization with grass pollen.