Observation of an interstellar line in absorption against the cosmic 2.7 K background is an unusual phenomenon. The intensity, Iv, of a line generated in an interstellar cloud, with homogeneous excitation conditions, is given by where Iv,bg is the intensity of the continuum against which the line is observed, τv the optical depth of the line, and Sv the source function, which is the Planck's function at the excitation temperature Tex, i.e., Sv = Bv(Tex). Observation of an interstellar line in absorption against the cosmic 2.7 K background, obviously, implies 0 K < Tex < 2.7 K, which requires rather peculiar physical conditions in the molecule, generating the line.