Absorption by N2 of the Lyman-Birge-Hopfield (LBH) bands in the far ultraviolet dayglow has been calculated using a line-by-line synthesis of individual bands. The effects of overlapping rotational lines and the combined magnetic dipole and electric quadrupole nature of the electronic transition are included in the calculation. A band transmission function which can be used in the radiative transfer theory of any band in the system is presnted for rotational and kinetic temperatures from 200 to 1000 K. Analysis of rocket observations of the LBH emission in the 1250--1400 ¿ region of the dayglow indicates that pure absorption by O2 dominates self-absorption except for the (6,0) band at 1275 ¿ where the horizontal slant intensity is reduced by 20%. The effect is much less than given in a previous analysis based on a band optical depth approximation.