In a double-blind study of 33 patients with bronchial asthma or chronic obstructive pulmonary disease, of whom 17 had proven atopies, the results showed a distinct diminution of the mean IgE level in the atopic patients at the end of 1-month treatment with Broncho-Vaxom, though not attaining statistical significance (trend with p less than 0.10), compared to a slight increase under placebo. Simultaneously the mean IgG level increased slightly in the atopic patients under Broncho-Vaxom and remained stable in the patients under placebo (p less than 0.10). The IgE and IgG values of the non-atopic patients in both treatment groups remained practically unchanged. Considering the small number of patients in each group, and the clear tendency towards a diminution of the IgE level, it may be concluded that the administration of Broncho-Vaxom to atopic patients does not lead to an increased IgE level but, on the contrary, seems rather to favour a decrease.