Glycogen levels in the tissue samples obtained from carcinomas and normal sections of human lungs (26 patients) were studied by measuring the infrared band intensity at 1045 cm(-1) due to glycogen. As an internal standard peak, the band at 1545 cm(-1) (amide II) was chosen, and the ratios of these band areas (A1045/A1545) were compared with histological classification and differentiation of tumors, The glycogen level in the carcinoma tissues was significantly higher than that in the normal tissues (P < 0.01, n = 26). Further, the ratio of amounts of glycogen in the carcinomas and in the normal tissues for adenocarcinoma was higher than that for squamous cell carcinoma (P < 0.01). The increased degree of differentiation of the squamous cell carcinomas appeared to be correlated with an increase in the glycogen level. These results suggest that comparison of glycogen levels in the tumor and normal section of human lung may be used as a differentiating parameter for abnormality and histological classification of tumors. The present Fourier transform-infrared spectroscopy (FT-IR) method may become of wide application for studying various tissue samples.