Two different cell lines sharing the multidrug resistance (MDR) phenotype were investigated for 8 months by means of Fourier transform IR spectroscopy on cell smears. We studied (a) a human leukemic doxorubicin-sensitive K562 cell line, from which a doxorubicin-resistant K562 cell subline was subsequently derived; (b) a Chinese hamster LR73 drug-sensitive line, subsequently transfected with the expression plasmid pDREX4 containing the mdr1 gene, to produce a multidrug-resistant LR73 subline (MDR-LR73). The sensitivity of Fourier transform IR spectroscopy has allowed differentiation between sensitive and MDR phenotypes among the above lines, even in double blind studies. The MDR phenotype is characterized by three combined features in spectra: (a) a decrease in the intensities of the amide I and II bands; (b) a shoulder on the high wave numbers slope of the amide I bands; (c) a shift toward the high wave numbers of the amide II bands. Furthermore, computational treatment of Fourier transform IR spectra (deconvolution and Gaussian curve-fitting techniques), has evidenced, in MDR-K562 and MDR-LR73 cell sublines, a conformational change involving the same protein in both sublines. It is hypothesized that the protein implicated in the conformational change may be related to the MDR phenotype.