The aim of this study was to compare the efficacy and safety of a monomethoxypolyethylene glycol (mPEG) modified grass pollen mix allergen preparation (mPEG-gm) and a partly purified grass pollen mix allergen preparation (gm) in hyposensitization (HS), evaluating both products at two dose levels. Thirty adult patients with allergic rhinoconjunctivitis were allocated into two treatment groups based on their sensitivity to conjunctival provocation tests (CPT). Treatment was given in a double-blind manner. The starting dose was 20 BU and was approximately doubled weekly up to 20,000 BU the first year and 120,000 BU the second year. Skin testing and CPT were performed before treatment and at each dose level. All patients reached 20,000 BU the first year. Twenty-five patients continued the second year. Twenty-one of those reached 120,000 BU (9/12 on mPEG-gm and 12/13 on gm). The frequency of general side effects was reduced by about 50% with the mPEG grass mix compared with native grass mix. A significant improvement in the conjunctival sensitivity was found in both treatment groups the second year (120,000 BU) but not the first year (20,000 BU). Seventy-eight percent of the patients in the gm group and 50% in the mPEG-gm group improved by CPT (not statistically significant). The skin sensitivity was reduced after 1 year at low dose in 69% of the gm-treated patients and 33% of the mPEG treated patients. After the second year at high dose levels, the skin sensitivity decreased in all patients.(ABSTRACT TRUNCATED AT 250 WORDS)