Fifty sets of twins (thirty monozygotic [MZ] and twenty dizygotic [DZ]), of whom at least one twin had a history of atopy, were investigated by means of skin tests, RAST and IgE determinations. The concordance of atopies for MZ was, with 56.7%, significantly higher as compared with the DZ with 20%. The mean values of the log serum IgE level differences for MZ and DZ sets of twins was not statistically significant but the percentage distribution of the log serum IgE level differences showed a closer correlation of the IgE levels among MZ. The MZ sets do not show a better concordance in the specific reagin production, as measured by RAST or skin tests, than the DZ sets. These findings seemed to indicate that although the tendency to IgE production is genetically determined its specificity is governed mainly by environmental influences.