- Fourier transform infrared (FTIR) spectroscopy has been used by chemists as a powerful tool to characterize inorganic and organic compounds. In this study, we examine the potential of FTIR microspectroscopy for early evaluation of therapy efficiency. For this purpose, we examine the effect of acyclovir (a known antiherpetic drug) on the development of herpes simplex virus type 1 (HSV-1) infection in cell culture. Also, we examine spectral changes in lymphocytes obtained from leukemia patients after appropriate chemotherapy treatment. Our results show early and significant spectral indicators for successful infection of Vero cells with HSV-1. Treatment of these infected cells with increasing doses of acyclovir reduces clearly the spectral changes caused by the infection in a correlation with inhibiting the development of the cytopathic effect (CPE) induced by this virus. Also significant and consistent spectral differences between lymphocytes from human leukemia patients compared to that from healthy persons are obtained. Treatment of these leukemia patients with appropriate drugs reduces significantly these spectral differences in a correlation with the improvement of the patient's clinical situation. It seems that FTIR spectroscopy can be used as an effective tool for early evaluation of the efficiency of drugs.