Up to now the effect of the modulated thermal self-emission of Fourier-transform spectrometers has been investigated for emission measurements only. But this instrumental radiation also influences Fourier-transform absorption spectroscopy in the mid-IR when the Moon, a hot blackbody, or even the Sun is taken as a radiation source, e.g., by causing small negative radiance values in the center of saturated absorption lines. For our experimental investigations, a blackbody that can be cooled down to liquid-nitrogen temperature was constructed. Measurements at different temperatures of the blackbody and for different optical configurations in the detector port of the Fourier spectrometer as well as transmission measurements of gas cells are used to examine the statements above.