The concept of nonmonochromatic modes of the radiation field introduced recently by TITULAER and GLAUBER, is employed to give a quantum mechanical description of the interference between nonmonochromatic light beams produced by different sources (e.g. lasers). The result is, in fact, that previous calculations by the author et al. in which each light beam was idealized as an excited state of one monochromatic mode, practically unchanged apply also to nonmonochromatic modes. The previous formulae describing the interference, on the other hand, are reproduced for the special case of (nonmonochromatic) modes corresponding to finitely extended beams of plane wave type, and, thus, are founded more exactly.