The major allergen of birch pollen, BV45, was isolated and conjugated to 2-o-methoxy polyethylene glycol-4,6-dichloro-5-triazine (mPEG). The molecular-weight variant of 6,000 daltons of the activated mPEG was used in an antigen-specific regulation of IgE response. In these studies 200 genetically high IgE responders (CBA/Ca) female mice were intradermally immunized by the purified BV45, with the use of various adjuvants. Four different sets of experiments were made. In the first experiment 50 mice were immunized on days 1 and 15 with BV45. This was followed by another booster dose of the native BV45 or the modified mPEG-BV45 in Freund’s incomplete adjuvants (FIA) on day 72. In the 2nd experiment, a similar procedure was performed except that booster doses were injected on day 135. Subsequently on day 136, BV45/FIA was intradermally injected. In the 3rd and 4th sets of experiments, 2 groups of 50 mice were daily immunized by intraperitoneal injections of 10 successive doses of BV45. On day 33 a dose of BV45 or mPEG-BV45 respectively, was intraperitoneally injected in each mouse. This injection was followed by other 10 successive doses of BV45. In experiment 3 no adjuvants were used, and in experiment 4 aluminium hydroxide gel was used as adjuvant. The mice immune response was assessed by analyses of the serum IgG and IgE levels. In all experiments higher IgG concentrations were shown for the immunized mice as compared to the non-treated control animals. Administration of booster doses of BV45 or mPEG-BV45 induced increases in the IgG levels. As shown in experiments 3 and 4, the mPEG-BV45-treated animals gave higher IgG responses than those treated with native allergens. Although the results of the experiments did not show a prominent decrease of IgE, we do not exclude the possibility that modified birch pollen allergens could be superior in hyposensitization therapy. The results obtained suggested within certain limitations that modified birch pollen allergens gave better results in modulating the IgG and IgE synthesis in mice as compared to the non-modified allergens. These observations are partially in agreement with papers earlier published on mPEG-modified allergens from honey bee venom, grass pollen and various tree pollens by other authors.